Power Infusion started as a Disc talent, and it should revert to being a Disc Talent.
Problem#1: Shadow currently has 3 cooldowns, none of which line up with each other: PI, Voidform, and Shadowfiend. This is simply too much to keybind. Remove their access to PI, and - instead - give a haste bonus to one of the existing cooldowns.
Problem #2: Shadow often can’t use PI for themselves in a raid environment. It does’t feel good to have to “give up” a huge DPS boost to another class. Sure you can get the 2-for-1 PI talent, but that requires an investment of points.
Problem #3: It doesn’t fit the class fantasy of Shadow.
For Holy… I’d take it a or leave it. Holy already has a ton of keybinds + cooldowns; having PI too is just more stuff you need to keep track of. To me it just makes more sense for it to go back to being Disc only.