The ritual wouldn't affect most of wrath's units,just the kinds that the barbarians would normally have. I tried timing Nature's Revolt for just after the avatar arrived in my last game, but the horsemen wiped another evil (Illians) civ just before I finished it. (I, Sheelba of the Clan of Embers, and Hyborem were still at peace with the barbs. All the other civs in this game had alse been evil civs, and were destroyed by the horsemen.) It didn't have any effect of the numerous Wraiths that Ars had spawned, nor the countless Chaos Marauders that I had summoned near my rivals boarders to wipe them out without declaring war.
From the Python code it seems clear that the ritual only effects unitclasses Axeman, Hunter, Scout, Warrior, and Worker. If the Barbarian state had more advanced units (whether from teching, starting in the renaissance age, enraged units, or the Avatar) they would not be changed.