Okay so which fullbacks in the world of football make things on their own? Can't think of many. Valencia is a RB who plays on his side on his own as our RW tends to cut in the middle be it Lingard or Mata. He has the physicality, speed and stamina to run on his own up and down the wing all game, he very rarely is out of position and almost never make mistakes these days, he's always back to cover and don't remember the last time he made mistake really... that alone is awesome.
And he's not bad going forward but what do you want from a RB? he can't dribble players like wingers do it, so many times he's really there alone but the way he dribbles is a very safe and good way of dribbling for a RB, because he really can't lose the ball there. And he absolutely never loses the ball when in possession, he may paly a bad pass but he's never dispossessed, his covering of the ball is absolutely fantastic. And he's got decent shot.
I think people should look at all his qualities what he adds to the team and not look at him as a winger anymore ffs. Mourinho would never like his rightback to try things and lose the ball in advanced positions. That's kinda how Young likes to play a little riskier but he's a better dribbler with a good turn and he's still playing it somewhat save, just wait for the defender's mistake and get past him or play the cross than dribble him.
Not every player player is Marcelo. Valencia can be a bit frustrating but it's not his duty to primarily create, he plays the way he's very solid 7/10 RB and slowly builds on that, he's waiting for the defender to make the mistake and ready to make use of that, he's nothing like comfortable, he gives his all all the time, he's just there on his own so he doesn't have many chances to exploit the right flank, while being solid at the back.