I will preface this by stating that, just like everyone else here, I am not nearly as good at evaluating talent as a college coaching staff.
Chastain is a good player. At the 1A level of the IHSA he is absolutely dominant. I think this is the part of the problem. Everyone sees him playing against 1A talent. I played basketball in high school at a 1A school. If you would have seen Chastain next to me, he would have looked like Michael Jordan. The overall talent at that level is just really bad.
I haven't seen him play a ton, but I watched both of his televised games in the state tournament. I think he has the potential to be a solid player at a mid-major. I wish SIU, my alma mater, would offer him. He just doesn't look like a Big Ten player to me. He might be able to develop to a point where he could play 10 minutes a game as an upper classman on a Big Ten team.
Of course, I could be completely wrong about all of this. It wouldn't be the first time. Given his offer list, though, I trust what I have seen with my own eyes.