The biggest flaw in Simmons game is that 33% of his shots from 3-10 feet, which is the highest percentage I've ever seen since I've been looking at the stat. He makes about 42% of them.
In other words, he relies on bad shots like floaters way too much. This probably happens because he is not willing to shoot a mid-range jumper, and instead barrels into a defender and throws up a prayer.
This is a really bad habit that can be broken, but I'm not sure he has the work ethic to do it. Right now, he only has two finishing moves - a dunk/layup and a crazy floater. If he has to take away some of his crazy floaters and replace them with mid-range jumpers or kickouts, his offense will degrade.