Why trolls so OP!?

[MikiTheBerserkr]

Very good health, and damage, still faster than dwarves, healing each turn, and a chance to ignore daytime -25% bonus!!!!! And they only cost 13 gold and are very easy to level up!!!!

[Ravana]

Balance is faction based not unit based.

[otzenpunk]

Actually their damage is pretty mediocre compared to other melee fighter units. And their defense is kinda low in many environments.

[dwarftough]

Damage-wise grunts are better than trolls. As otzenpunk noted, their damage is mediocre and defense is lower. I will note that, although occasionally troll spam may work well, there are pretty obvious setups where it can't work. Consider drake, for example, drake units have nice impact res and burners will burn trolls during the day pretty easily (low defense except on hills/mountains has its effect). Also consider loyalist spamming calavrymen, they have so good impact res that trolls damage against them is ridiculous, especially during the day

Summarising, trolls are nice but spamming them works only sometimes

[MikiTheBerserkr]

Damage-wise grunts are better than trolls.

Not that much

As otzenpunk noted, their damage is mediocre and defense is lower.

Grunts have even lower defense.
And trolls still regenerate, that's extremely OP!

[dwarftough]

Grunts have even lower defense.

Aren't you confusing defense on terrain with resistance? Yes, trolls have 20% res against blade and impact (and -10% against arcane), but their defense on terrain is definitely lower: they have 30% on flat, 50% on hills, 40% on forest, village, castle and only on mountains 60%. While grunts have 40% on flat, 50% on forests, hills, 60% on village, castle.

Not that much

Still significant.

Also 1 lvl trolls are slower (4 mp vs 5 mp), this together with lower damage makes them more suitable for defense. For offense orcs more often buy mass grunts.

[otzenpunk]

Grunts have even lower defense.

???

[Krogen]

Pretty much all that's been said. Trolls are not OP. Even their lvl ups are just okay, nothing outlandish. They have more hp than grunts at lvl 1, but beyond that, grunt line surpasses them at that too while maintaining higher damage and gaining a ranged attack at lvl 3.
Also if you pit Troll Warrior against Royal Guard in 1v1, about half the time the RG will win, even though it's 'speciality' is an extra movement point at lvl 3, let that sink in.
Trolls do exceptionally well in battles where solid lines of units collide and surrounds are not possible, so they gotta soften each other up before they could kill anything. (Rarely happens in player vs player games.) Regeneration really comes in handy in those situations. But even then, so many possibilities to even the scales, like healers, or chargers, berserkers, or leadership for boosted damage so kills become possible. Also every faction has one or more options against which Trolls just naturally don't do very well, except for Knalgans maybe, so that makes countering them even easier.
Also here is the thing, regeneration only matters when the unit doesn't die in a single turn. Against focused attacks by multiple enemies that have the damage output to take out a unit per turn, the ability doesn't actually accomplish anything. (That's a good mindset to counter them, focus on a single troll with everything you can, kill it and leave the rest alone. Then move on to another one in the next turn. This way they get very little out of their regeneration.)
In multiplayer, trolls are only essential against Undead, and that's purely because of the impact damage they bring to the table. They are also good against Knalgans generally and especially with some long-game strategies, but they are only worthwhile in very specific situations against Loyalists and Rebels, and are straight up bad against Drakes.