For me, it's his lack of defensive ability - he frequently fails to stop crosses on D, and is out-of-position at times given his attacking instincts (but mostly recovers due to pace when this happens).I got slated in the post match thread for saying he gives me the shits. It is precisely the sloppy passing he does at random intervals that causes me this concern, I want him to do well but I am sorry he worries me.
His greatest assets are pace (not sure if he still has this following multiple injuries) and his whipped crosses into the box (he's starting to get back into form in this regard).
I'd like to see a full season with a fit Mendy before making up my mind for sure.