Opinion4fun : Overrated vs. Underrated !

Here's 3 for each to start

Underrated
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Eliwood
Considered inferior as a red cavalry compared to Xander and Eldigan due to defense, yet he can withstand all the mages the other two cannot and he has a more useful speed stat, with a speed boon and either Fury or Darting Blow, he can consistently double a large number of units at a power of 51 thanks to Durandal unbuffed and Swordbreaker lets him out-duel all other sword units 1 on 1 including Ike and he destroys his childhood friend Hector as well.

Sophia
Triangle Adept and the red Raven Tome is the most effective with her stat spread more than any other red mage with the +atk/-spd iv spread since her speed is so low she wastes less there and with the enhanced triangle effects, all the doubling colorless and greens will barely dent her and her excellent mage tank stats

Jaffar
He is constantly overshadowed by Kagero and her infantry effective dagger even though so many teams run more armor and cavalry lately than infantry that her dagger doesn't seem to be as effective as a more general devbuff dagger. Give him Windsweep or Watersweep and L&D or Darting Blow with Savage Blow and he can strike, debuff and chip damage a bunch of enemies at once risk free for your tank s to slide up or nukers to finish off.
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Overrated
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Reinhardt
Almost every Tier 17+ Arena team is running him and every team attacking the Arena has some kind of green tank/mage built specifically to bait and counter or kill him on initiation. It's just becoming redundant and he's warping/defining the meta.

Hector
*See Reinhardt*

Ike
Much less commonly seen (at least for me) but also vastly vulnerable to the blue mage meta we have currently, but he's also much slower than other popular reds (since ideal teams seem to be red, green, double blue) he fights for that 1 slot but any other Swordbreaker red will kill him since he usually drops his breaker for vantage and if he doesn't, he's more likely to lose the matchup still because the other will likely be faster than him anyways (Swordbreaker Eirika for example trumps him)

Lucina is the most overrated hero in feh. She is not that good.

In my experiences:

OVERRATED:
- Lucina: Don't get me wrong, I love Lucina on my team. But I feel like other sword units can do just as well, and most have better bulk than her

- Takumi: Average attack with pretty decent speed, with pretty decent defensive stats as well makes him a lot less of a threat than he use to be. Nothing really stands out in terms of stats

- Effie: The only reason why I put Effie here and not Hector is because Hector has DC, meaning he can be a lot more versatile. Effie is quite easy to take out with any mage, regardless of color type

UNDERRATED:
- Marth: While he has less of an offensive presence than Lucina, he makes up for it with his pretty good bulk. With Fury he can reach over 30 defense and Res into the mid 20's. There have been many times where swapping out Lucina for Marth helped me complete some challenges

- Felicia: Undoubtedly the best mage counter in the game. She won't do anything against the Raven tome users, but they are quite rare. There have been arena matches where she fought more than one of the current popular mages and still pulled it off

- Oboro/Lukas: This might sound like another weird one, but I have been looking into them for some time now and they have potential to have a really good W/L ratio with the right build. I don't have them but I'm sure there are people out there who would claim them to be sleeper units

My point of view in tier 20.

Overrated :

Nowi = Since Reinhardt is so dominant Fae Julia and Nino are more common and destroy Nowi too easily. + She can't tank Reinhardt and falchion users still destroy her if she doesn't have TA3 and swordbreaker. She's still very strong against physical red non falchion users and physical blue units but not overwhelming like before.

Ike = Fae is a new popular unit that can destroy a not prepared ike or damage him pretty badly depending on her build. Tome users are more and more popular and can one shot him. A lot of units can double him if he isn't +SPD. He is still a strong hero against physical green and red units. Fast red units can kill him if he doesn't have swordbreaker thought.

Kagero: Just not as good as before because we have more and more flier/horse units.

Underrated:

Fae: Good hp, def, res and atk. Her speed isn't too low when built well and can have a distant counter. She's not a SS+ unit but she perform really well in his niche (killing blue, Reinhardt, low res green units) and a little + is her high BST. Her counter are fast red units and Julia.

The most overrated: Azura, Ninian, and Olivia.

Overrated:
1) Nino - yes, she's amazing; but Cecilia with the extra movement and cav buffs makes her a little obsolete in my opinion.

2) Ike - His def/res spread is less useful than Ryoma in an archer-light meta; and he can't take the kind of movement type buffs that Xander can.

3) Falchion wielders - fantastic for Chain Challenge and Tempo Trials. Simply outclassed by inherent distant counter swordsmen in arena.

Underrated:
1) Fae - a great counter to the blue mage heavy meta. Not an inherent user of a distant counter weapon but relatively easy access.

2) Firesweep builds - a fast, hard-hitting unit with Firesweep bow or lance is such a good counter to the distant counter + vantage combo that is so common, particularly with dancer support

OVERRATED :

Hector : He is a very good and powerfull unit and probably deserve to be in high tier. But people kinda turn him into the holy graal unit but he actually have plenty of counters.

Nowi : She is still ranked pretty high while there is actually no reason she is considered better than some of the other dragons in the current meta. Falchion users are far less dominant and anyway you just need a good blue unit in your team to counter them. If this unit is nowi she'll be able to kill them with adequate build but will still take good damage in the process. On the other hand the new anti-dragon unit is Julia that I see almost as often as Falchion nowadays and she absolutely destroy Nowi.

Ike / Ryoma / Xander :
They are very strong indeed but still have a lot of counters. The fact they have built in distant counter is nice, but due to their very low RES they often fail to mages even sometimes green ones ( Fully buffed Nino can often kill them when she is in desperation range).

UNDERRATED :

Raven tome (especially red ones) :
Raven tome users are often overshadowed by blade mages but can be extremely efficient due to their ability to take bye themselves a huge part of the rooster while taking little damage in return.
Red mages are considered less good in the meta, and I would agree for blade mages but not for raven tome.
Blue blade mages can kill most blue and red units which make them very scary, but for raven tome the most common blue one, Robin M isn't strong enough 1 shot most blue or red without a breaker (and he can't run both sword and lancebreaker. )
As red raven, Sophia, Lilina and Sanaki offer enough damage to kill most green while still being able to wreck havoc on sword users thanks to SB.

Tiki / Fae :
As I said before, no reason Nowi is considered better than thoses two in the meta right now.
Tiki Y is actually a beast, she is the fastest of the 3, reaching a decent 36 spd with fury and the good IVs while having a great physical and magical bulk.
Fae is a bit less fast but her typing and high res allow her to tank most of the current threat such as Rein, Hector (magic damage totaly kills him) and in general most blue and a big part of green units. She gets desotryed by falchion, yes, but then again she is green and shouldn't fight reds much.

Debuffer daggers (Saizo, Mathew, Jaffar, Felicia) :
All those characters can run a debuffing type of build that can allow them to soften the ennemy team for your own team to do the kills.
I use a saizo with smoke dagger, poison strike and savage blow. He can really weaken the ennemy team while charing his Ignis and then he can just OHKO a huge amount of units.

Mention to a few more units :
There is units with unconventional stats that are considered bad but can actually turn into interesting unit if you are a bit creative.
Recently there was a video of a jagen reaching 47 Res with cavalery buff and a specific build (and +4 from berkut's lance).
I have been using a Hawkeye as a blue wall and it works pretty good.
Selena with a Wo Dao and fury turn into a pretty strong melee unit very hard to kill and that can actually kill you very well when the special is up.
Sheena is overshadowed by Hector but she has amazing overall bulk with the ability to tank both physical and magical damage from green and blue units.
Subaki is an interesting tank with pretty good def and spd.

It all depends on which units you use the most (thus which ones you can beat rather easily)
Every time I see a Takumi or Ike at the arena, I feel happy for the free kills (at most maps) but I realize they're actually super good, its just that they don't get super buffs or rare tricks to pull.
At this point the meta is blue, so most red units seem "not that good" as they could be (arena wise)
As for underrated... healers in general, perhaps? I see most people post teams without healers, yet I use them in pretty much all my teams (F2P rank 19 every map/challenge complete at every difficulty)

I'm going to push back a bit and say that I don't think Reinhardt is overrated. In my opinion, he's terribly boring with only one viable role...but I consider him the single most dominant unit in the meta currently.

Hector (Overrated): I love Hector. He is a beast. But he isn't the force that he was preinheritance. He's still the best armor with his combination of QR2, Distant Counter, and an open B slot...but you don't really have to plan a team to counter him anymore. I'm not saying he's weak...because he is very strong...just that he's overrated.

Kagero (Overrated): Like Hector she's still highly valued from her preinheritance glory days but she's lost her valued niche as teams have become more equine in nature. She remains the strongest dagger user...but that's really not something to brag about. She just can't stand up to non-infantry in any meaningful capacity and she even lags against optimized infantry.

Young Tiki (Overrated): Much like Hector and Kagero, Young Tiki is still highly rated from the preinheritance when - aside from access to DC through Lightning Breath - she's very much on par with Tobin. It's also not great to be a Red magic attacker in a meta filled with high resistance blue units. She was one of my first three 5* units and she carried me through the early days but I think she's faded more than is usually acknowledged.

Ike (Overrated): Maybe it's just because my Ike has bad IVs or because I've shredded most of the Ikes I've faced...but something seems off about his current standing in the meta. I know he's a popular Lord and for good reason - he was awesome in his games - but I don't think he's quite the same powerhouse in FEH.

Eliwood (Underrated): I've long considered Eliwood the wimpiest Lord in all of Fire Emblem. He couldn't get anything done in FE7 without Hector and Lyn helping. But I think he is extremely underrated in FEH. He's an offensive powerhouse - probably my favorite melee nuke in the game due to Durandal and having three movement.

Male Robin (Underrated): I'm not sure when his fall from grace happened... He was quite well regarded preinheritance but somewhere along the way he became considered a middling unit. I'm not sure why... Inheritance did him a lot of favors by giving him TA and breaker skills. Maybe it's the growing irrelevance of any colorless unit not using a bravebow? But in a meta full of fast, powerful blue mages I think this steady defensive one makes for great counter to physical attackers and is underappreciated. Not saying he's top tier but just that he should be get a bit more respect.

Overrated:

Hector: I think everyone has covered this by now, but a unit who gets dumpstered by the red Dancer, never mind watching him explode into a million pieces even looking at the likes of Ryoma or Lucina isn't S+ tier in my book. I get why he's obviously amazing in Armor Emblem, but outside of it you have to babysit him so much due to the amount of units that dumpster him.

Ike: He's too slow to double up much of anything and gets destroyed in a single blow by a good chunk of the mage cast before his Distant Counter is even a factor, I have a friend who has a 5 star Ike collecting dust in his hero box because he found him so underwhelming in practice.

Takumi - Inheritance exists now. Takumi's Close Counter was nice when the game first came out, but compared to the nuking capabilities Bridalia, Quadsuna, and Quad Gaius possess, and compared to the utilitarian builds with the Firesweep Bow, he just doesn't offer as much as he once did.

Underrated:

Fae - Others have brought this up, but the list of characters Fae dumpsters (if you use one of theLightning Breath+ build) is practically a who's who of the top of the meta chart. She'll even often take out a Xander foolish enough to attack her. Falchion users dumpstering her doesn't matter when Lucina ORKOs every Green to begin with, and Chrom and Marth are rarely seen in Arena.

Julia - I know she's considered S tier, but she is without a doubt the most annoying defensive character in Arena thanks to her Tome, attack power, and high resistance.

Overrated:

Hector: An armored unit with Distant Counter, good strength, defense. Can very EASILY get destroyed by the Elite 4 of Red Mages.(I still laugh at the fact how the loved/hated Tharja can take this "immortal ax lord' down no problem.) Not to mention a spd buffed Ax user that uses a Hammer two-shots the guy. Also he's an armored unit and since he can only move one-space at a time, it's pretty predictable to see where he'll go.

Effie: Yet another good armored unit. Though she's not the that great. True she has the highest Atk in the game not to mention Wary Fighter, but that can't save her from easily getting one-shot by a really good Green Mage or like what I mainly use a ax wielder with a hammer (Talk more about that unit in underrated.). Effie is just no serious threat to me.

Reinhardt: One of the best Blue Mages in the game. Can one-shot units when engaged even it they have an advantage against him with low resistance. What happens when a unit has actual good resistance or when melee units can get close to him and possible two-shot the guy? The guy's speed is probably his main weakness, exploit that and he's possibly screwed. On a final note, why is it a crime to use veterans in the Arena which gives you less points when you can easily get rid of them with the right counters, but using Reinhardt get's no points deducted?

Underrated:

Frederick: When you think of green units you may raise from a 3* to 5* and put in the/see in the Arena what do you think of? For me 1st it's Nino obviously, then followed by Cecelia mostly cause Horse Emblem, heck I'll occasionally see people using Cherche in the arena. It's just a shame that only in a blue moon I'll see Frederick there. He's like a Hector on horseback and more mobility for me (minus the DC and unique weapon). Good Atk and Def stat okay spd (which is enough to double an unbuffed Reinhardt and kill him) and trash res. He can go toe-to-toe with Hector (Double him if buffed in Spd) not to mention one-shot Effie no problem. Helps buff my Blade tome user with Fortify Def. Not to mention is blue mage check when they get close enough in range. True, magic is his Kryptonite (Again, why is it a crime to use Frederick when he's not that hard to kill with magic?) but he's one of my most reliable green units. I just wish more people used him.

Titania: I lay awake asking myself how is Titania only 3.5/5 on this site when she's one of my 1st/best Reinhardt counters. Sure her Atk isn't the greatest but with her TA weapon it kinda doesn't matter too much since blue mages can barely do anything to her AND she could probably wreck a majority of top tier blue mages. Not saying being a Reinhardt check automatically makes you a top tier unit, but I feel like you deserve a little more respect than just 3.5/5.

M.Robin: I don't have a Linde/Delthea or Reinhardt (Or his sister). So when came to a good blue mage, M.Robin's been there for me. Before SI he's was useful for me in Arena Battles, and during SI Arena he's now my Red tome/ranged Clear unit check thanks to my TA Quick Riposte combo. Any really good Red tome almost dies with little effort when they engage since 3/4 of the Red tome units can outspeed him in battle. Takumi can't do crap to Robin anymore, and Kagero can't either. (And when she can damage him, she can't finish him off, but Robin can return the favor.) Also I gave mine Ignis than Bonfire when SI came out. Almost always Ignis ready when he's done with a clear/red unit.

Olwen: Though I don't really own one, I've used her a few times in the Voting Gauntlet. And I got to say, I like how she quad units unlike her brother which makes shine more than him in my opinion.

Gods I'm sorry this is so long.

I would like to say Reinhardt is overrated, but not having a counter to him makes arena runs way more difficult.

I think Hector, Ike and Kagero are overrated pretty much for all the reasons some of you have mentioned, so I'll focus on underrated units.

Eliwood: his atk can get quite high with horse buffs, +atk, deathblow + durandal. With swordbreaker he destroys greens and reds. Also, he has good resistance.

Soren: before Sonya, not a lot of people was talking about Soren. He may be overshadowed by Nino, but he can be a great alternative to her. Soren can get to 51 atk/41 spd with the right boon (mine has those stats, I think it is +spd).

Lukas: his defense is really high. With ignis/bonfire and quick riposte he can tank pretty well and deal insane damage.

Lloyd: with fury he gets to 51atk/38spd +2 if you attack someone with full health. Slap desperation and he is pretty good. He also has really good res. I know he has a lot of competition, but he is quite solid.

Hmm... Where to start...

Overrated;
1. Reinhardt - It's pretty obvious why he's overrated, his dastardly Dire Thunder tome and his high attack stat makes him notoriously difficult to bait without a green mage like Julia to end him of his rampage, and his power spike is even more so with Deathblow and the help of Moonbow+Quickened Pulse seal. His movement as a cavalry and being a 2 ranged hero allows him to cover basically half of the damn map.

2. Azura - Fuck this bitch for ruining my perfect deathless streak due to unpredictable AI.

3. Hector - His vantage shenanigans usually get me killed, because of the fact he survives the first encounter and ends up having Bonfire ready to destroy me.

Underrated;
1. Olwen - I honestly like her, like it's sad that people tend to overlook her for her brother who has better damage output overall, but Olwen has very good potential due to her high speed which is enough to run a Quad striking build with her own Dire Thunder tome.

2. Jeorge - He's in my opinion the coolest looking archer in the game, his base weapon when looking at Takumi's is basically better and more useful(Warding blow 2 vs Pass 2) even though his resistance isn't nothing to write home about, he's a pretty damn good archer and it's sad to not see him being brought up more.

3. Navarre - Easily the coolest looking GHB, he has very good potential to be a sweeper but he tends to be more favored for heroes with built-in distant counter like Ryoma or Ike.