The Undead themselves aren't much to speak of, but the Death Knights... few can compete with them except for Paladins of their own level. They chop through enemy heavyweights like firewood. The fodder too. Far more useful than Ghost Dragons.
The most important units in the Necropolis are vampires, liches, death knights, and skeleton archers (if there is a good necromancer).
In battle, owns Necromancers do not lose troops, but gain them, unlike Sylvan Elves (who find it difficult to replenish losses). It makes no difference to them which town they capture; everyone will be turned into skeletons. At high levels, Necromancers cast powerful weakening magic, and knowing Berserk or Hypnosis... only a Warlock has better SpellPower. If Logistics is developed, a Necromancer can amass huge armies in just a few weeks of game time.