[ano]

Ermor - because all their priests reanimate undead, hordes of undead.
Shinuyama - because of diverse and powerful magic and good troops
Mitclan - because of good troops and great mobility, good magic, flying communions, good summons and other things.

By the way, I think that Mictlan should break into blood, be it unthematic or not. As well as Bandar Log. Or at least one of them. To both nations blood gives simply great summons and for Mictlan - cheap blood hunters as well. This is how I would play if I were in Usurpers team, most probably.