I started to think that the most unbalanced town (pre-Equilibris) is Necropolis. The other ones seem to be quite balanced (maybe Haven is somewhat weaker).
I am not very experienced yet (I started to play HOMM4 some 4 months ago), I am able to beat most scenarios at the Advanced level and Necropolis is the only town that allows me to beat them on Champion. It is enough to follow the template: get Vampires ASAP, develop a Necromancer to GM Necromancy (usually it happens about 10th-12th level), then add a Death Knight and go for conquest. Usually it is not necessary to get reinforcements because of evergrowing army of Vampires.