I've said it before, and I'll say it again: Rollers are like Team Fortress 2's w+m1 pyros. intimidating at first, but easily dispatched once you've mastered the arcane art of walking backwards and shooting. They seemed overpowered in the Testfire because of how inexperienced everyone was, allowing the rollers easy results. Once people get more used to playing, however, they will be relegated to a (quite vital) support role, because for all their power, they can't win a direct fight.
So no, I am not worried the roller is overpowered. At all. Very useful, powerful if they get the drop on you, but not overpowered.