The difference is layout. That would be the main obstical to overcome. Rita is designed to have quick and snappy changes in direction. Going forwards gives riders the chance to see what is up ahead and brace themselves. Even then quite a few people still compain of discomfort as the transitions occure with little warning.
When going backwards you would find it impossible to see what is coming. The result? An unbraced bashing around. Not pleasant
Also, as Sam has rightly pointed out Rita accelerates at a much higher pace. LSM rides generally pick up speed gradually at first before reaching a point where the vehicle can accelerate further. Much like in a car how you would generally speed up gently at first until the car has enough momentum to accelerate faster in a shorter period of time.
Rita on the other hand releases a sudden burst of built up energy, literally grabbing the car and pulling it forwards very quickly in very little time. It's one of the main advantages of a catch car based system I believe.
When Rita launches your whole body is pressed back against the seat, and your head against the headrest unless you try to keep it forward (Which is also difficult given the rapid acceleration
) The seat and headrest are there to support your back and neck. Without them your body would continue to travel back, forcing your back to stretch the wrong way, and possibly causing serious injury.
If the train was to be reversed all you would have in front of you is the two OTSR ribbons, which are far from comfortable. You would be pressed fully into them, causing them to dig into your shoulders, and your head and neck would have no support whatsoever. You'd get severe whiplash to say the least