His pace saves him when he is out of position alot of the time. He lacks serious concentration the most common example being he misplaces simple passes and most likley once a game one of those passes will result in a good counter attack for the other team.
Hes obviously been good since he came having been part of 2 titles but he also can't cross a ball, while its fine when hes playing a more withdrawn role it would make such a difference if we had someone who could cross a ball properly from the right hand side. Although Mendy hasnt played much you can clearly see the difference when he plays on the left attacking wise.