Balance in Heroes 5.

It is foolish to think so, because the Dungeon wins through unit strength, but they are few and expensive, and the loss of units greatly affects the army's combat effectiveness

Well, I don't quite agree with that; it's unlikely that without casting the Dungeon could withstand anyone, the small number of units certainly plays a part. That is why I believe the Shadows win primarily through the power of magic—Meteor Shower simply mows down everyone and everything, although in 2.1 I think they nerfed it a bit; the meteors stopped hitting quite as hard.
As for the balance, the Necropolis stood out in all parts and was, in my view, the strongest, and five is no exception. All the others are more or less equal, except perhaps the mages were made slightly weaker than the rest, or maybe I just don't know how to play them.