The majority of injuries and deaths caused by rapids rides are the riders doing the wrong thing and falling out. The minority is people falling out because something was wrong with the ride or raft (tube inflation, water flow, too much weight, etc)
Meanwhile the Dreamworld incident was purely a clusterfuck waiting to happen and it really seems like that was the only rapids ride in the world with the safety flaws to allow it to happen.
Generally if you sit and stay put, you'll have no problems. You stand up and dick around, you're gonna have a bad time.