I believe Kishi would made Yata Mirror/spirtual item working like that:
- If attack is weaker then Sussano Itachi is using while wielding Yata Mirror then Yata nullify attack and Sussano stay
- If attack is stronger then Sussano Itachi is using while wiedling Yata Mirror then Yata still nulify that attack with no problem but after it It got dispersed along with Itachi's Sussano forcing him to have some cooldown time. As Sussano user also seem to have small cooldown time after Sussano got destroyed and want to reform it.