No matter how much I don't play the fifth game, Necropolis almost always loses, whether they are controlled by the computer, by me, or by a friend.
Personally, I think they are equal in strength, and often Necropolis wins against Castle. It all depends on the player.
The undead have an advantage: skeletons. If you take care of them until the battle with the opponent, you can accumulate so many that the humans will be in trouble.
The undead also have good mages. Yes, they are created slowly, but they attack in an area. And everyone knows about vampires. Phantom