In my opinion, the racial ability of Necropolis has just as many drawbacks as the racial abilities of other factions in Heroes...

The racial ability of Necromancers could be quite good, and perhaps even overpowered, if it weren't for the meager amount of dark energy it generates for long games. While other racial abilities only become stronger in long games, the influence of Necromancy on the game actually weakens. In other words, Necromancy is only effective at the beginning of the game, where you can raise hordes of skeletons, for example, and win because of it. Later, in the late game, 3-4 specters raised per week no longer make a difference, while an elf can easily strengthen his sworn enemy to the point that no amount of Necromancy is enough to raise enough creatures to counter him, given his racial ability.