there was one performance where Mizuki was off on her part, and then i believe also the mic recording that came out early on (which i believe was from a concert) made me cringe through her line, though granted i didn't think anybody besides Reina sounded good... but Riho's attempt was better than Mizuki's attempt yet people always focus on Riho.
not that i have anything against Mizuki or her singing, just noting people's lack of judgment against Mizuki while there is no hesitation when it's Riho.