Trouble is with Sane, is at times he does not track back. Now if you playing with 4 3 3, if he don't track back for oppo wingers or full back you are leaving Zinchenko massively exposed. Pep has spent an age coaching world class players to play a system that everyone know. The possession of play is because the player with the ball expects someone to be " there" it's second nature. It's the same when the players don't have the ball, they are trained as a unit to battle for the ball and track back.
I'd agree that he doesn't do the donkey work that would help zinchneko. The argument could be made that if you had a better lb (defensively) then it wouldnt be such an issue. But nowadays it's only really cb and dms that are expected to be really good defensively.
Can't argue with the results pep gets. I just don't think Sane is bad as such. He simply doesn't fit imo