Fire attack buff and affliction resist says hi
Considering the aim for elemental attack is to get roughly 200% from eidolons anyway, the other 100% should come from other sources right?
Plus amaterasu comes with a def buff and healing. The only thing those two really share is B-frame debuffing. Other than that, they're practically two separate kami.
Plus, while it is a long cooldown, the damage nuke is still welcome cause it can easily hit 6, maybe even 7 digits considering you should be debuffing when possible. Amaterasu has a consistent nuke with blind, while Yamaraja has one that works like hercs but with debuffs. If there's high debuff resistance anyway, then both of them are pretty much using a dead skill
Amaterasu is one I overlooked to be sure, but Yamaraja is no slouch either (especially considering buffs and debuffs other strong kami can have).