People are overestimating the yata mirror, because they forget to think of something called chakra levels. Itachi using the Yata mirror could probably defend against anything, because that is the way Kishi told us the shield works.
But he hasn't infinite chakra. Maintaining Susanoo seems to be needed for using the mirror. If SO6P keep making attacks, that cannot kill himself upon repulsion, at some point Itachi will simply run out of chakra and thus he cannot maintain the mirror.
The only thing that was ever said to be invincible was Itachi's Susanoo, and not Itachi himself. Don't forget manga facts.
While we don't know, where these items come from (the three relics of Japan), I think it is safe to assume, that they hold same value as the items created by the so6p, in other words they are indestructable as stated by Kurama.
My personal belief is, that they were created by the elder son of the sage, and that only Susanoo can wield them, because of the manner in which they are related to Susanoo in the Japanese myths. Likewise it was probably the elder son who created Susanoo along with the special requirements to obtain Susanoo, due to its great power.
