His big problem is that his defensive options are lackluster. His fastest move is frame 5 so he has a lot of trouble with opponents that spam short hop aerials right in face. He lacks an amazing "get off me" move, and this gives him a hard time vs characters like Sheik, Diddy Kong, and occasionally Sonic. Furthermore, many characters can take advantage of his fall speed to punish him harshly.
However this is offset by Roys amazingly good ledge game and offensive pressure. It can be hard to get out of Roys pressure once it gets going. There is an important distinction between going "in" and staying offensive. You want to keep your opponent just at the edge of your range until you open them up, then you go in for grabs and sweetspots. Most people assume Roys design means he needs to be right in the opponents face all the time, which is actually detrimental and leads to people thinking his design is inherently flawed and that he's bad.
These are the main things holding him back right now.