Board Thread:Theories and Speculation/@comment-25132842-20180320052324/@comment-25132842-20180321201436

LegionZero wrote: Madara's strong chakra would mean his MS would be proportionately strong and thus, hurt him

That makes more sense. The mere fact that they have strong chakra doesn't mean that their Mangekyo doesn't damage their eyesight. After all, Sasuke had strong chakra, and his Mangekyo almost made him blind.