have you tried roux on bis cubes? is sounds like the M moves on 5x5 and up would be harder because the slice layers are much heavier than on a 3x3.
The slice moves aren't that hard, there is just the risk that some of the layers won't turn, so that's why I don't use it. Also, Roux is very inspection heavy, and it is harder to just solve without inspection.
That being said, I still sometimes use the MU U perms on 5x5, even though I know the RU ones.