I actually think he’s “theoretically” really good on defense. He has quick feet, and his positioning and decision making are really good. He also tries his ass off.
But he’s just small. So players shoot over him or overpower him. Same with Isaiah Thomas. Their defensive fundamentals were great but the size was the handicap.
Conversely I think Giannis has pretty poor defensive fundamentals but he’s just massive and freakishly athletic so he’s effective. If you watch closely, his footwork is relatively poor and he gets cooked a lot but his size and athleticism allow him to recover and still affect the play
If by deep you mean 5 deep lol