Hm undead AREN`T overpowered ???
Lets check this out :
skeleton archer best leadership/dmg ratio in game, and i think they are strongest 1st lvl unit,
ghost/cursed ghost/ghost pirates completly sick for enemy phisical dmg dealers, no need to buy them they grow by themselves,
vampires/ancient vampires no retaliation, life drain, no CRIT hit (sick unit)
black knights with shard of darkness most OP unit in game,
necromancers, great for +1 morale undead, and aoe attack for lots of rage, good suport unit,
bone dragons aoe poison attack, against all living/demon/dragon units great dmg, cause low resistances.
All undead have pretty low leadership per unit. I know only way to keep no loss is evlin, but it`s requierd for ranged units vampires/ghosts keep their own numbers with life drain skills, but on other hand evlin have highest resurrecting % in game.
As you mentioned human units are great picks, but i stoped using them because they bored me like you. Paladin is great suport with phantom spell, inquisitor also, rune mage is overpriced and his rail gun attack isnt that great because his low speed and attack, horseman is sick with warrior counterattack skill. But comparing human with undead i must say every undead have better leadership/dmg ratio then their human counterpart.
And human item sets are weaker then undead.
And if you look for overpowerd unit/combo try solo asmodean in red sands mod
it`s so easy, it`s not even funny
i tryed many builds and hero combinations, and they all have up and down sides it only depends what you like in this game and there is so much to like.I only wish that they somehow balance all choices in game like all archer build is totaly sick (triger, paladin-archer-elf-hunter-5th unit by choice or moldok/rakush paladin-blood shaman-goblin-goblin shaman-catapult), full undead army with moro dark 2 undead sets and 3rd lvl evlin. Why there isn`t some interesting full elf build, or some skill to boost mele fighters like quick draw in might skill tree.