People think it's OP because it's a one hit kill if you run someone over with it. If you can get up close to someone with it, they're done.
I wouldn't say it's balanced perfectly, but I also wouldn't say it's nearly as OP as some people seem to think.
I can see it being the core of many teams, but Roller will probably end up needing a pocket for support. Being limited vertically and easily dispatched by ranged weapons (if they player can actually aim) balances it pretty nicely. These comments are with thoughts of high level play. With random lobbies, Roller will probably dominate that clusterfuck.