My biggest worry about him is the KU game. Most have probably noticed, but since Willis has been starting and getting more minutes, Cal has been forced to move Poythress. Willis isn't strong enough to defend the five spot, nor does he have an offensive game suited for it. That has forced Cal to play Poythress at the five instead.
Although it's been working out okay so far, with KU that means that arguably UK's best interior defender (Poythress) will be wasted on KU's worse offensive players (Lucas and Traylor), and arguably UK's worse defender (Willis) will be tasked with guarding KU's best everything, offensive player (Ellis).
Ellis has an all around offensive game. He can shoot the three, put the ball on the floor and drive to the basket, and he has a very, very good offensive post game. Not going to lie, Willis having to guard him scares me. Hopefully Cal will change things up and tell Willis that he's just going to have to man up and guard the five position, allowing Poythress to guard Ellis.