Actually it is pretty easy. I agree that it was stated to be invincible. Indeed it was backed up with the smallest or close to smallest feats like physical charge of hydra, paper bombs and also sworld attack. Now it is arguable about yata mirror was used against Kirin but most likely partially formed only. However what bothers me about mirror is if it is connected to chakra use of susanoo being part of it...
Another interesting point is that either yata negate effects of attack like making it not happen at all or if it still happend but yata make it ineffective against wielder.
If we go first approach(paper bombs threw at yata still exploded instead of being rendered useless). It could imply yata was used against kirin so Itachi was not affected but kirin caused explosion which could cause Itachi to fall down loosing control of susanoo as it was shown maintaining focus is important.
If we go by second approach(not supported by manga) then well there is nothing to point out.
Lastly Kishi isnt as straightforward and direct mangaka as Akira whom he is looking up to. In other words Kishi has only left hints and clues of something being ineffective by not making character using it in situation where it would be called of.
See Itachi not unleashing Yata mirror against Chibaku Tensei. i know that there will be excusses for Itachi not needing it although shown otherwise or blaming plot. Really? Then yata mirror getting hyped up is also plot do ehat ?. Also finally Kishi not bothering to bring yata back through Sasuke to prove it cements it further.
