[WraithLord]

HoF statistics clearly support your claim.

Strangely enough Ermor didn't seem so strong in early game when it couldn't recruit mages and all it's undead armies were insta banished by Pythium.

I suspect that secret of Ermor's power is more subtle. It may have more to do with Psychology than with raw power.
Perhaps the cost-reward ratio of attacking Ermor makes it an unattractive target. Perhaps it's something else.

I don't know. I do know that MM is hvy and that I am content to settle for this one time MP experience with this unique and interesting nation.