i don't think warriors and defenders are too weak. assassins are rather weak, though. warriors/defenders are just great in the early game, but once you can build good troops, you don't really need those heroes any more. defenders keep a little niche that noone else occupies (tanking single strong monsters such as dragons that oneshot trained units with the "overpower" trait), but warriors just suck later in the game if they don't have very good items (and no, double burning axes aren't good enough to make them not suck)

commanders and mages are quite the opposite- they are rather weak early on and become more powerful in the later stages . there are exceptions, of course (mages specced for support instead of damage are very good throughout the whole game, for example)

Compared too who?

Since you are using damage to compare (and not including anything else):

Mages -> Yes, they are underpowered compared to mages, but technically if you wanted to you can make your warriors into mages too by getting spell books (either from faction trait or by buying from allies' shops)

Commander -> No, commanders are definitely the weakest (since you are basing this solely on the amount of "personal" damage they can do)

Regular Troops -> Well "mid-late game" troops is a very broad category.  Generally, at mid game my warriors are not obsolete.  The late game depends....  but then again I don't play the game with a tunnel vision on damage.

My primary stat (the one I value most, but NOT the only one that matters) is initiative.  That is why I play a custom Amarian race with the Decalon trait, and by mid game every one of my heroes is at least an air mage and a watermage.  With enough fire shards, and everyone being hasted, and the enemy being slowed I usually get two to three free attacks against the enemy before they can react.

So to answer your question... no I don't think warriors/Defenders/Archers are underpowered.  At least not the way I play the game.  The way you play is probably different, and I understand that; just giving you my perspective. 

I don't find warriors underpowered at all. I actually prefer warriors to mages in the game. Give your warrior a good sword, shield, and some armor then go down the Lethal tree till you have reap 3, then go down the counter attack tree until you get your +1 counter attack with any weapon and you're unstoppable. You can literally march out into the middle of the enemy forces and let them hit you while you kill them with counter attacks. By level 7 or so, I'm killing deadly monsters with my warrior by himself. I don't even give him an army because they just get in the way and slow him down. My level 8 warrior 1v1d a Slag earlier and cleared an entire Troll Shaman camp at level 9 by himself. So no, I don't find warriors underpowered at all.

For a long time i was going experience boost > sweep > lethal tree with a warrior.  Walk up to a group, sweep, cleave the next biggest chunk, clean up.

Now I'm trying out the mage/summon/horde.   Its great, but a lot slower.   A good warrior was a 3-4 round combat.  Mage takes longer.

I don't like having my sov be defender/assassin/commander - seems too weak.  Although I like supporting heroes of those types.

Yes. I am talking about Champion who heavily based on gear.

Most obvious would be Warrior, the Warrior only work fine in early game if without epic gear.

Perhaps this is what the game design intend?

I don't think warriors/defenders/assassins have weak skill trees, but they are definitely a lot more gear dependent than the commanders and mages. Get them some good gear and they hold up fine in the late game. Fail to get good gear and they become only good for back line support.

Assassins seem to be the only class who are noticeably underpowered in my experience. 

I usually regret choosing that path whenever I do -- would love to see them made more interesting and/or desirable.

Decimate is a weak overpower.

Lethal needs to be a +% damage bonus, maybe +40/+30/+30

As for assassins, they need +1 dodge/lvl, +1% crit chance per level, and a trait or bonus that every 20 pts of accuracy they succeed by they get +1 attack.  Also I'd add in a dagger and bow mastery line, or allow them to dual wield a dagger for more damage.

that's really a non issue. the game is over at this point unless you refuse to finish it. none of those repeatable techs should factor into balancing, since they are just there for the people who want to play "one more turn" and don't care much about "winning" the game.

also, the tech benefits trained units more than warrior champs, since their combined attack value will probably be higher than a warrior's, even if the single warrior has the best items in the game and is fully buffed (including spells such as growth and giantform)

Right that's why i'm against giving warrior too many attack modifier by percentage. Because warrior are never meant to be one man army, they are for 1 vs 1, like in the original FE, mage is for going against massed number of weaker unit than the 1 vs 1 unit.

Personally i don't find anything wrong with warrior. The only class i feel lacked is assassin, but i don't want to make judgement too early, i will wait another month after trying a couple assassin experiment.

Here is a suggestion for warrior for the dev and to please people that complaint about them being useless. I'll bold the suggestions. The inspiration of this suggestion is taken from Eador MotBW.

Make warrior the only class that can change weapon on battle. They seems to have traits that gives bonuses to most kinds of weapons. Especially spear, axe, and sword.

With the ability to change weapons on the battlefield, they can change to weapon which give them the best advantage for a certain condition on the battlefield.

For example, at the start of the battle, we should use swords because swords has counter and at the start of the battle we start in defence mode, so less one shot or one hit dying thingy for our warrior, and the warrior still give some damage to his/her attacker due to counter which only sword weapon has.

Then when the warrior's turn finally comes, he may want to change to spear or axe to use AoE attack, if you look at other classes, they doesn't have bonus to that weapons, so warrior will be much more efficient in using that weapon, this give the warrior the role of the AoE damager like mage, but warrior is for AoE Physical Damage, mage is for AoE Magical/Elemental Damage, and this gives warrior more play style that is unique only to them,  a warrior can play his way as a counter attacker or axe wielder or spear wielder, depending on which one gives them the best advantage for a certain condition on the battlefield.

True, still i don't want to see that "imaginary state of the game" to be ruined because of that overpowered warrior which has 200 or 300 atk, because, you know this game has many things that exist for that state, dragons for example, they are only available in that state. Actually, 30% if not 50% of all LH players loves to play in that state. Not all people care for winning the game as soon as possible and not all people are hardcore gamer, we better play RTS if we want to be competitive and being hardcore or playing a strategy for the purpose of winning only, all 4x always fail to address this issue, hence why 4x is just for fun, look at Master of magic (MoM) which give FE and LH inspiration, most people plays MoM for that "imaginary state of the game", but somehow such a state in MoM blend nicely with normal winnable state of the game, and even in MoM there are limits to overpoweredness.

Running around with a Warrior who has around 30 initiative and 40 attack with Reap 3 and you'll laugh at how easily that champion can solo entire armies of slow troops like Oppressors. 

Assassins should be able to use dual daggers (and like double axes, i mean two separate daggers like a Leech Dagger and something else). That would increase their attack power and make them a little better melee champions when they don't crit. I'm not sure about two 1h swords, that might be a little too much, but they would be sacrificing defense (no shield).