Sam Vecenie had one of the Piston podcasters on his show recently. They did 20 minutes on the Pistons and talked about Stewart. They praised him for improving his shooting. But they said he has no gravity, he doesn't draw the defense in so he really doesn't create spacing like you would want.
They mentioned the fact he can't attack closeouts as being the big problem. You can really play off of him on defense because when/if he gets the ball, he will not move at all. So you can close out on him hard and he isn't capable of punishing you by putting it on the floor and driving to the basket.