Beasts do enough damage to overwhelm block, making them good vs. humans.
Humans have one thing the others don't — BLOCK. Combined with Enchantress, she can transform enemies and make short work of them. Thus, attackers aren't as important as defenders. Keep her alive and you conquer everything. Use evade, block and % def auras. Humans are good vs. retribution demon builds, which are slow and get transformed, unless they are high attack with crit aura.
Demons are versatile, but I think of them as favoring retribution. Dark trees and autumn stouts punish anyone with a high attack/life ratio, destroying archers and attackers. Battles take long enough that Witches gain momentum. Lifesteal and heal technically equate to damage, since that life gets reflected when lost. Demon retribution is very good vs. armies reliant on raw stats like beasts.