I've only ever flown one aircraft with an ejector seat; the JP. Never had to use it, thank goodness. I was told however, that back injuries were less common in the RN. Because of the catapult launch and cable recovery, the pilots were strapped in much tighter that the general RAF pilot would be. One from my South Cerney course was demonstrating the rig, which was, I believe, a one-third charge. There was a mis-fire, and he loosened his straps while they sorted it out. Sadly for him, the seat fired, and he damaged his back such that he couldn't proceed to pilot training.