Yes spinal compression is a definit problem if the sholder belts are attached at any more then a 45deg angle, but 4, 5, 6, ect. point harnesses and roll cages (more then 4 point) are dangerous for street use. Cars (or atleast ones built from roughly the late 70's and on) are designed with crumple zones and by adding bracing like roll cages takes these crumple zones away or makes them less effective. They are there to absorb the energy of the impact so that your body doesn't. The same goes for seat belts. Racing harnesses are designed to have very little give where as factory belt systems are designed to keep you in the car, but they give far more and are really to allow you to move in an impact but slow you down (kinda like the arresting cable on and aircraft carrier). If using racing harnesses without other safty gear (i.e. neck braces or hans devices) severe neck and upper spinal injurys are highly likey due to the fact that your body is kept in one spot yet your head is allowed to moved freely. The amount of energy and force generated in a 50mph head on collision is more then enough to tear your spinal cord.