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Vampires are definitely awesome. Liches, however, are a more debatable topic. They are almost like mages, only mages have more attack power, while liches have more defense. Considering that they are the only ranged unit for the Necromancers and will therefore be heavily targeted, they need more defense. And their area-of-effect attack is a great thing. It's just a shame that they also damage their own units.
Necromancers develop quite well (+attack and spell power), but I don't like the fact that they have:
firstly, one less slot for general skills (necromancy is cool, but there are no other skills this specific; although necromancy is offered to Warlocks, it's still better not to take it).
secondly, their troops don't combine well with troops from other races. If, for example, you can take dragons from a captured castle instead of the battered Ruins for the Wizards, and with the Knights and their innate leadership, you can combine troops from many different castles, then you can't take anyone to join the undead. And if a Necromancer hero switches to troops from another city, all his necromancy becomes useless