He can score so efficiently at a high volume within the Bucks system which is
designed to spread the floor and put the ball in his hands. Not sure he would be that efficient on a team like the Bulls or Spurs who don't have three point shooters for whom the lane is often clogged.
I think he has potential to be one of the best of all time, but right now his ability to score is more or less limited to driving the lane and putting back offensive rebounds. He's got no shot and limited post skills.