I don't think C frame def down debuffs are inherently less accurate, I think that it's just for a long time cthulhu is the only C frame def down debuff and it's pretty inaccurate because it is not single target but debuff all and those tend to be less accurate. SSR amon is good but in general thanatos is a better hime as the other spells are better and it is largely easier to find an A frame debuff to hit 50% def down.