Because there are a thousand and one ways to circumvent Reflect. Remove it, throw Joy or Shank in there to buff your damage dealer’s defense against itself, use “slow burn” type characters that won’t burst themselves to death, and so on and so on.
You know what there isn’t a thousand and one ways to do? Dodging enemy abilities.
Q. E. D.
I’m not asking why Sisu’s Mim disk is better; I’ve already said I agree with that.
I’m asking why her Mushu disk is “garbage”.
And apparently your answer is that there’s other ways to handle reflect, and so I’m guessing you also consider Fairy Godmother’s Mim disk to be “garbage”, correct?
Yup. But the other option is a very small situational speed buff, so it’s still the better option for Fairy Godmother.