I think Simmons is a really good player. I have no problems with him winning ROY. He has limited shooting range which is no small flaw but almost everything else is off the charts good. Court awareness, length, athleticism result in a rare combination for someone as big as him.
I suspect his shooting will improve but even if it doesn't, he can still be a 6-10 Rajon Rondo.
With Tatum, let's see how much muscle he can add this off season. He had a terrific rookie run, no doubt. Similar to Simmons, even if he is not able to add the needed muscle/bulk (as Simmons needs to add shooting touch), he will still be very good. The floor is very high for both these guys as is of course the ceiling.