Huns, Mayans and Aztecs all underwent some clear nerfs, and celts were already almost as good as huns in aoc if you ask me.Why celts?
What about Magyars? A strong Scout Cavalry with knights later on with +2 attack? Seems good with me.
Japs wont quite make it near celts just because of bloodlines. Their m@a rush is still pretty much the same, and the problem with the attack faster bonus(33% infact), is that it doesnt make them better vs archers by much since m@a spend most of their time taking damage from arch rather than hitting them. I prefer celt fast moving m@a that can actually catch archers/vils, or the cheaper goth m@a with extra damage vs buildings.
Main weakness of Japs has been the lack of useful bonuses after the initial pull is gone, like with mongols. Bloodlines solves one of their shortcomings though.