The difference between Evans and Williams is that Willaims is pretty much the go to guy there, especially with Rogers out for the season. The coaches have no choice but to make him the #1 option in the passing game (I doubt that T. Streets is their #1 guy, but I dont watch their games, so I'm just speculating).
Evans is not even listed as the #2 here, being behind Reed, so the opportunities for him are less, especially when E. Moulds is the #1 guy. I think once Evans is put into the gameplan some more, and is offically the #2 guy, you'll see the things he's capable of. I think this is all just a case of how the coaches use these two players and not necessarily that Evans is a bust in comparison to Williams.