I remember Zetsu saying the Mirror is imbued with all chakra natures, which (I believed that) implies only as a counter to ninjutsu. It's been shown to counter taijutsu also, but I do believe 8 Gates Guy can break through it if he could break through Onmyoudon (which is, in essence, the same thing). That being said, it doesn't appear that there is a counter to genjutsu with the Yata Mirror. However, Sasuke's Susanoo was able to nullify the light of Infinite Tsukuyomi, so I guess the Yata Mirror should too, since it is an extension of Itachi's Susanoo. So all in all, I think Kaguya can probably just shatter it with sheer force (back to possible taijutsu vulnerability)