So by your definition of patchy, which Spurs player isn't patchy? And which elite PL LW isn't patchy?
You mentioned Mane.
You're right that's he's patchy. But when he's in bad form, he still impacts games. He will still take players on, still make runs. When he's off it he will just miss more chances than he'd like, mess up passes, and generally be frustrating.
When Son is off the game, sometimes I forget he's on the pitch at all. That's a different, more worrying, type of patchy. If you're not on form you can still be a danger and a presence, whilst doing some of the basics right.