Most of the concerns people have had with his defense are due to rotations back to the corner of the zone which is the weakness of the zone itself. If a team can make a crisp pass to the wing, then it forces Moses to come up, and stay with that guy longer for the defender from the top to rotate over. If the wing offensive player is quick, he can swing the ball to the corner for an open look unless Banks rotates quickly which is hard. The real issue when it is happening is allowing too easy passes to the wing where you would like to force passes that allow for quicker "handoffs" of defensive assignments and let Moses start rotating back before a pass can be made.
He also gets beat some off the dribble but it is usually by smaller guys, wings and guards, which is to be expected to some degree.