If your back and bum are sore you're probably not sitting right, have tightness and/or muscle weakness that should be addressed if you don't want it get worse
Sitting on a ball can make your posture right, you know how your chair is all adjustable so you can get in a good position relative to your desk? Balls don't do that. You might not have the strength to sit with good posture on a ball, least not for extended periods of time. I'm not saying it's bad for you if you were to do it, just there's a reason we're not all doing it