What are some units with unusual/bad IVs that you've come to appreciate?

I don't really use that many 5* exclusive units, so any "odd IV" combinations that I'm not pleased with can easily be remedied with getting a 3*/4* unit with the IV set that I'm more at ease with.

I do have a +Def/-Spd H!Henry that I have that I tend to use fairly often, so I guess that's one.

My second 5 star after marth: Hector. I have been using him ever since I didn't know what IV's were at launch, and I still have him on my main team even if I don't have any more merges of him. He's Hp+ Atk-, which is not an ideal IV for Hector, but he somehow makes it work. I gave him bonfire so that any double attack would activate it, and also renewal so that Armad's effect would still work. He is an excellent panic ploy user because of his high Hp. I have him in my defense team, my main story team, and everything else with Nowi/Reinhardt, Ryoma, and Elise/bonus unit. Also, his other stats are made use of, even speed, since he has an attack bane. I'm really glad that I am able to use him as my best green unit to this day.

Ny! Ltakumi. +def -res. With glimmer hes really good at baiting frail range units in aa and ohkoing them on the counter. This is thanks to his good high decent stat giving him good mixed bulk. He can even take on some slower bulkier units like ike on occasion.

A few, one that comes to mind is my +def Sakura. With Spd+3 A slot, she has 33 speed, 28 def, and 30 resistance, which makes her surprisingly tanky as a healer and great for training.

Mine would be winter robin. Love the character. In the winter banner I got a +spd -atk. after that I made him an offensive and defensive unit with Berkut's lance res refined and +3 atk S skill with QR3 B slot. On a legendary banner he was on I got one of his perfect ivs in +spd -res, which to this day I still find it hard to merge into.

I have a -Hp +Res Dorcas who was incredibly powerful for me. I briefly considered foddering him, but then he turned out to be one of my strongest Green units for a really long time and one who I invested in a fair amount. While I hope to get a +Res -Spd one some time so he can use Infantry Pulse, these IVs have worked great (I run him with Warding Breath and Aether).

Another one is my +Def -Hp Hinoka, who I didn't expect to be that useful, didn't build for a while, and considered foddering for Hone Fliers. But I built her with just HinoSpear +Spd, Repo, Blazing Wind, Fury 3, and Hone Fliers, and she's already insane. 42 hp, 54 atk, 38 spd, 32 def, and 27 res. o3o

I got a +Atk / -Spd Elincia that puts in mad work. A lot of people would probably dismiss the spread because she loses on the ability to quad, but from my experience, she only needs the two hits to kill everything. Plus, you don't need to worry about any counterattacks.

+Def/-Atk Charlotte. I really struggled to build her into something she's supposed to be good at, and then the Refine came to my rescue. Managed to really make use of that +Def superboon and I would go as far as to call it one of her better boons.

As to units, Henry all the way. CC + Owl really shows his strengths.

One of my earlier 5*'s was a +SPD/-HP Hector. He was a staple. Eventually I pulled a +ATK/-HP variant and switched over to that one. Cue me losing in PvE content because all of a sudden he is being doubled by stuff that he wasn't before. Turns out 28+6 speed back then was plenty to avoid PvE doubles from mages. So he could bait and counter kill even red mages. Not anymore he can't.

I have even seen a Hector build that was speed focused. Windsweep and phantom speed. Go ahead. Bait him if you want. Not quite on topic since I switched him out, but I actually miss my +SPD Hector!

I've got severall of those.
Sonya (-atk + res)
Even though atk bane hurts she capabale of doing a lot of damage thanks to moonbow and wo dao tome, extra res is great for tanking and ability to land ploys so i gave her fury renewal and spd ploy.
Lute (+res -spd)
She still hits 40 spd with darting blow not to mention her res helps a lot in tanking, baiting and landing ploys.
Tana (-atk +spd)
I dont mind atk bane since she has atk tactic wich bumps her atk back at base value, i have a lot playerphase units so she was forced into mixed phase thanks to having +7 when initiated on, then add fortress def, seal atk/def 1 and guidance. Great support, tank and duelist all in one Tana.
Inigo ( -atk +res)
I actually was glad to see him until i found out that bane, still i decided to build him as raventome mage and i never regret that, he has like 34 atk with weapon equiped but that matters not, thanks to TA3.
H!Henry (+hp -atk)
At first i felt helpless since i wanted him in reverse iv (-hp +atk). I found a way to fix him however, spd refined gronowl +death blow 2, renewal and armormarch. I have double goad armor so he usually gets +8 to atk and speed, not to mention his owl tome effect.

I just want to add that in most of the times ivs can't kill unit, they can cripple it's performance but even then you can fix it with TA and breaker skill if any other ways won't work.

None. I have too much pride to use units with terrible IVs

Back in the first few months of the game I pulled a +hp -res Lucina who was a standout unit for me for a long time, though she's since been outclassed. I also had a +def -hp Fae, which look like great IVs *now* but at the time everyone was all about increasing her attack or res or even speed and considered def her dump stat, and I was like, no way, she can mixed tank like this and it's *great*.

These days I have a +hp -res Halloween Henry, and those sound like terrible IVs since he's supposed to be a res tank, but it turns out that he still has ridiculous amounts of res even with a bane and the hp superboon helps bring his otherwise subpar hp to more reasonable levels. I like him a lot, although he's been a little outclassed since I pulled Valentine's Hector (my first and still only Hector ever) and I need to figure out what to do about that.

Great question for discussion

The only 5* Sothe I pulled is +HP -def (or -res? his res is higher than his def now.)

Seems close to neutral, but I'm finding that I quite like the extra HP. He rarely survives more than one hit anyways, so may as well take the extra mixed bulk. If I'm not mistaken I survived the blue mage in the current F.Takumi GHB that spawns after turn 1 on the right hand side with 3HP.

I have a -spd A.Tiki, +def I think she has, and I promoted some other copy instead of this one hahaha silly me. The funny thing is, on several occasions she's actually managed to avoid a double. Non-spd bane might be better than I initially gave it credit for.

I pulled H.Henry on a legendary banner -atk +def. At first I was simply disgusted with his attack, and then I got raven+TA and now every time I see B.Lyn and Reinhardt in the same team in AA I'm actually happy because I get to use him. He generally takes a bunch of 0's and 1's from them except on specials even if they both have merges and use hone cavalry. I gave him noontime to try to keep QR2 active if Reinhardt strikes first with a quickened moonbow, so he sometimes has issues finishing off B.Lyn, but I'm starting to think I can solve the issue with glacies because it won't activate if Reinhardt attacks first.

I've been blessed with a lot of IVs that most people would kill to have (+spd on mages, +atk on Gunnthra and B.Lyn, etc.) but I'm actually finding that I have some issues with some of these "ideal" IVs. Dorcas I have +def -spd, and he's so slow that Myrrh can sometimes double him for a KO, which has caused me issues in PvE when I still used him for it. (His HM is close to maxed now so he rarely sees PvE, but it doesn't change this struggle he has.) S.Linde I have +spd -def, and I'm actually thinking about using a def refine to boost her survivability on a DC counterattack.

My first Legendary Robin was -Atk +Res. Used her as a ranged tank, and sometimes I wonder if that's not one of her better IVs for that. Adaptive damage on Expiration makes the -Atk matter a lot less, while the Res superboon does work. I ultimately merged into a -HP +Atk one I got later, but I still think -Atk +Res has merit.

+Spd -HP Lukas, and +Spd -HP Reinhardt. They reach 36/34 Spd respectively when prioritizing Spd (with Reinhardt easily being boosted by Hone Cavalry). Since their standard builds would not compliment their IVs, it was a fun challenge to build them.

When I got my Hector, I had no idea IVs existed, so I didn't know what they were at the time. When I checked them, it was -HP +Spd. I had no reaction, I thought it wasn't a big deal. For a long time, I used Brave Luci with that Hector to give him 42 Spd, and it was amazing.

Side note: I had Aegis on that Hector 'cause I liked to be able to take a hit from Red Mages (especially Sanaki) and then kill them. I eventually switched to Ignis, but I loved using Aegis.

I also got a +Res -Atk Ryoma. I noticed his Res stat with a Res boon and Fury is actually decent, So I thought I could use that to my advantage somehow. I haven't yet, but I probably can.

Definitely my -Atk, +Res F!Grima. Turns out that resistance was her superboon, making her defenses 40/30/29 which raises to 40/39 on enemy phase thanks to her A skill and Fortify Fliers. I slapped Quick Riposte and Moonbow on her and she was good to go. And considering that her attack focuses on the lower of an opponent's defenses, 45 attack isn't all that bad.

A couple:
Hinoka: +Res, -Atk. She was a non-focus pulled out of nowhere and her -Atk was a little off-putting, but I was determined to use her so I decided to build her for a more support based role. I gave her WoM for warp shenanigans and Steady Blow for some dueling potential. Then, her new weapon and refinement was released which boosted her usefulness in that role. Now she's a mainstay on my Flier teams.

W!Lissa: +Res, -Spd. Her stats are insanely tanky so I decided to capitalize on her +Res and gave her Handbell a Res refine and she ended up being a huge help for challenges as mage bait and a general tank. She also was kind of a gateway unit that got me into starting to use Armor teams.

+Spd Ephraim, either of them, is pretty dandy to avoid doubles honestly.