Balance in Heroes 5.

I'll say it again: quantity, not quality, is what matters!

But what about it? Can't any other race win by sheer numbers? Undead also have useful combat abilities – for example, draining mana or draining blood and regenerating! Or when zombies hit an enemy, they get covered in some kind of green goo, and their stats decrease!
It all depends on the hero; if you level him up to level 30, you won't need a large army. The quality will be so good that you won't even need a large army!