What's everyone's benchmark for a unit's stats?

33 res is the absolute lowest I'll go to run stat Ploy.

50 HP is the lowest I'll consider for running Panic Ploy and Infantry Pulse

That's about it.

Atk: Depends on the unit, but 52 after their weapon is generally my benchmark. But any unit at -Atk is unusable imo.
Spd: 42 or higher for offensive units.
Def: 35 or higher
Res: 35 or higher

I try to keep Attack at around 48-52. If they have ~55, a Fortress skill works great on them. It can be a little lower if the unit has a Wo Weapon.

Speed varies on the unit. Offensive I'd say 40+, Fast Tanks at 35-37, and Slow Tanks at 30 or less.

Defense I would like to be at 32+. Same can go with Resistance, especially with Ploys.

And HP depends, Offensive should have 40 or less, Duelists at 40-45 Tanks with 45-50, and skills that rely on HP at 47+.

47 atk I can work with
33 spd I can work with
Def,It varies if your 30 or higher I will consider it worth some investments.
Res,never really focused on it.

Good question for everyone to ask themselves. I tend to be willing to get quite specific depending on who I'm targeting, so for example, sometimes 45 or less atk will do it especially if I have a big special cooking.

atk: very dependent on what special is involved and who I'm targeting. Too hard to give a generalization except for some specific cases:
- squishy mages (and other ranged units): two strikes of 46-47 atk will usually do it even against merged opponents. 56-60 atk for OHKO, 64-65 if they're highly merged.
- Ayra, Karla, Mia, and some similar units such as Soleil: 67-68 magical damage for OHKO. More if merges are involved. Lilina and blue Micaiah, take note.
- Falchion OHKO vs Fae, Myrrh, flying Tiki, and some versions of Y.Tiki: somewhere around 50 atk.
- BoF or Summer's Breath OHKO except against blues: Y.Tiki +atk doesn't need an A-skill if she has the effect on BoF.
- 50 physical attack is not enough against armours without a very big special

Spd:
26-27 is a relatively safe number for avoiding non-forced doubles from mages like Julia or Micaiah (of course, Micaiah might brash desperation double, hence, non-forced doubles). It's also a fairly safe number for L.Hector, Myrrh, and wary fighters trying to block bold fighter doubles from slow armours (i.e. Zelgius, Amelia, W.Robin, etc. need to be accounted for separately)
32 is a relatively safe number for blocking manakete doubles except vs flying Tiki and fast F.Grima
39 is getting into territory where some fast units are going to have issues doubling you
41 is a pretty safe number for trying to double the same manaketes whose doubles you can block with 32 spd
42 is when fury Nino+10 might not double you.
45 is when very little can double you without bold fighter
47 is when the fastest +10 units I've ever seen cannot double you without buffs
55 is when you natural-double Karla. Nina can get almost this high with just a drive spd if she uses atk/spd bond and spd ploy.
15 is when you're dancing Xander.

Def+HP on squishy unit:
below 50 = don't expect to survive one hit
50-53 = marginal
54-56 = decent chance to survive one hit
57+ = likely to survive one hit unless weapon triangle disadvantaged

Def: depends on the thickness of your HP and whether you use guard. If you're trying to take multiple hits, guard or not, absolutely no less than 35 with 40HP. Armours and highly aggressive infantry often have 54-61 atk; duelists usually have 49-55; play accordingly. If you're trying to keep your guard for multiple battles, very minimum 48 def.

Res: if you're trying to dump (ex: Bartre, Ogma), 10 is good; less than 10 is very good.
If you're trying to tank, depends on how much HP you have, what colour you are (green tends to require less) and how much damage you're willing to take. I generally haven't had to think too much about this because I've always worked with the assumption that if you get doubled you're in big trouble, which is true for my merge projects.
If you're trying to ploy, depends who you're trying to ploy.
Res ploy: generally, 38 is a bare minimum, because you're probably trying to hit their res, so if you fail to ploy they really have you walled. Unless you just give up on those and say "I don't need to ploy those super-high res units because I'll double them easily"
Spd ploy: 34 is a reasonable number. Most super-fast units including fury Nino+10 do not have 34 res. Most melee units have significantly less than 34 res, and those that have more are usually slow.
Def ploy: 30 will sometimes do it, because melee units tend to have poor def. If you want to cover manaketes, 34 is safer.
Atk ploy: in most cases, 34 is fine, but 35-36 is safer for some cases vs armours. 38 if you want to hit Caeda and Clair (and more if you want to compete with their merges).
To hit Sheena or fury Felicia+10 with a ploy: 43 res. Have fun.

Panic ploy: 46 feels like a safe number except against armours. 55+ is preferred against armours. PvE, just forget it.

Infantry pulse: offensive units are usually the ones who want to receive it, and a lot of them don't even have 40HP without summoner support. 47 is a pretty safe number for your highest IP supplier; for stacking IPs on some lower HP units 43-46 is perfectly fine.

It totally 100% depends on abilities. 39 Atk is pretty miserable but I'll frequently use units with Atk that low if they've got other redeeming qualities and/or get a tempest trials stat boost.

From time to time I use my 36 atk Odin, 37 atk Niles, 39 Atk Leo, 37 Atk Matthew.

When it comes to units that do damage off of specials (Nephenee, Marisa coughcoughHenry), there's not much of a bench mark. But I heavily focus on Def/Res and another stat they use really well (Nephenee and Marisa with Spd).

For fast attackers, I'd say the same. 40 is the bare minimum. A lot of times I feel a little underwhelmed, so I go even higher.

And as for tanking, it's not just Def/Res you take into account. HP is also an issue. Characters like Nephenee and Selena Gomez don't mind having sub 40 HP all that much when their Def can reach into mid 40s and 50s rather easily. But then there's other characters that can still tank with around mid 30s defenses but pack at least mid 40s HP to back it up.

These are my considerations when I look at units for tier 20/21 arena so it assumes 5*+10 for these units and their enemies. I mostly use these units on other maps too, along with emblem teams, and various niche units otherwise.

Atk: Must be over 50 but prefer 55+ unless the unit has really good stats elsewhere.

Spd: 40 base is when I will invest into making a unit faster (+Spd, Spd refine, Spd/Panic Ploy ally, buffs, etc.). Otherwise I assume a unit will be doubled by fast units and account for it in stats/build.

Def: If I expect to get doubled (see above) 35+ and HP in the high-40's. 40+ if I want them to see multiple rounds of combat.

Res: Don't care if I expect the unit to only tank 1 hit from a mage (OHKO counter or high Spd). 35+ or at least 50 HP for tanking 2 hits.

Panic Ploy: 53+ HP
Other Ploys: 34+ Res
Heavy Blade: 55+ Atk (with buff/debuff support)
Flashing Blade: 40+ Spd (with buff/debuff support)

Tbh anything above 42 atk is fine with me as long as it's a unit who can double reliably (40+ spd usually). For defensive units, 30 Def or Res is a must for me to feel comfortable.

But as long as there are no skill requirements like heavy blade or ploys I don't really ask myself that question. I like to use all my (5*) units even if their stats aren't perfect.