There really isn't enough Dark Energy, so it's not worth calling Necropolis overpowered in the mid-game, especially since this is an endgame resource and you won't be able to raise many monsters of level 7 or 6 in the late game.
You should only raise the most necessary units of levels 1, 3, 4, and 5...
P.S. I'm not complaining about the Dark Energy...