Valencia is an interesting one, I was never a big fan of him as a winger as he lacks flair, I was always in the Nani camp. However as a RB I think he's fine up to a point, he's very solid and as
@Pexbo says, he offers a good out ball and possession retention.
The big issue is in our current system the fullbacks need to provide a threat in attack, you could say this of the modern fullback in general but when you use inverted wingers then that overlap needs to provide penetration and a good final ball, Valencia has been seriously lacking in the latter for some time now, and it's highlighted when when compared to the chances Blind and Shaw have created from the LB position in the last 3 games.