IMO his biggest problem is footwork.
Take Pierce, for example... Especially the KG-era Pierce. Good-not-great athlete, average quickness, good handle for his size... But his footwork might have been the best in the league for a small forward. He was always balanced and in control, and he was crafty enough with the ball to be able to attack an opponent leaning the wrong way with either hand. (Being strong as a bull and a knockdown shooter are helpful too, lol).
Young, as a counterpoint, is a comparable athlete but lacks the footwork, and that affects his game in many small ways. It's why he usually looks out-of-control on drives, it's a major factor in defending screens (his biggest defensive flaw IMO) and it can also impact his shooting stroke at times.