I used a Rock n Roll for a while. What was great was that you could see more than just in the hips if the rider "quieted down" after making a change plus standing up felt much more organic to the client. We only stopped when we started using cameras as it made plotting knee-path (2D system) impossible.

even a stationary trainer will waver a bit but I have found it can cause some small issue with the video but not too much since I also use a laser line to set the horizontal which is independant from the bike. I have recently switched to a Computrainer and it is rock solid. The last triathalon fitting I did the video showed how much the frame flexed. The amount and location of flex was actually quite telling. I would probably not buy that frame for what I saw. It swayed like a wet noodle and the guy was only putting out 250 watts in the aero bars. So the set up is interesting but it is hard to say which is the worse situation, the bike swaying/flexing or the trainer swaying.