Because you think he's performing the best out of a bad lot does not make his performance good just better than the others. For me NONE of the wide players are good enough, none provide enough, create enough, help in possession enough or help progress the ball well etc but I've been saying this since last season. I said it this summer. I'll probably still be saying it next summer.
In terms of costing us goals hmm yes I think he does. He averages around 30 touches a game, almost always the lowest of any of our players and sometimes even lower than subs that come on. So he's quite a passenger other than in those moments where he finishes chances. To some extent we are playing with 10 men when he's on the pitch due to his non involvement. That contributes to the times we lose the ball and can't regain control because we are playing with a numbers deficit especially if Son is on the other side also not really contributing. Son has a lot more touches but they aren't necessarily useful touches but he's at least somewhat involved in the game.