True, Black Zetsu called Itachi. But notice how he said that it was yata mirror and totsuka
blade in conjuction making him invincible. If Yata Mirror was unbreakable to anything, that
would be all Itachi needed to be considered invincible. Besides, he was damaged after Kirin
hit him. Every higher version of Itachi's susanoo was shown holding Yata. Saying yata didn't break
is suggesting V1 or V2 Susanoo can protect the user from kirin enough to survive.
So, the only real feats it has was blocking explosive tags. If it was shown blocking a juubidama or something,
then we can start talking.... Regardless, I'm still sure yata mirror is an extremely top tier defense. Probably
a PS level defense. But invulnerable? No.