Undead are the main offensive force of the Necropolis, as dragons are more expensive and don't reproduce in such large numbers. Death Knights can also guarantee the death of one unit (whether it's a peasant or a dragon). I always take them.
I don't know how others play, but my main force consists of skeletons and liches (maybe dragons towards the end of the game).
Undead are only used for intimidation...
Vampires are also good, but you can't rush with them as easily as before (they've been offended, poor vampires:( )
Ghosts seem like decent units only when they're on the enemy side, but my own ghosts seem to die very quickly...
