@vancexentan he only mimics mobility things so he can keep up with his patient. And not a full mimic, just the motions. So if he attached to bakugo for example, whenever bakugo uses his explosions to move in the air, he'd mimic the jerky motions, but can't use any explosions or anything. Same with super speed quirks.
Thats sort of the idea. It takes a long time to build that much charge, and if they don't use any other charge and save for that 100%, it gets rewarded by something very powerful that forces the opponents to run until it wears off. The alternative was turning the heal beam into a damage beam. I should mention kncokback forces still work on an uber patient so there is that.
75% makes them immune to CC and heals very fast, but they still take the damage that'd occur. So if they'd die by getting a car thrown at them, they'd still die. Just that the car would probably hurt them more since all that knockback force is diffused differently.