Ejection seats a rocket or a pyro gas generator driving a piston are pretty much the same in performance. Both get you out of the plane and above the tail so you do not get sliced into pieces. Limit is the acceleration of the initial push, too much and you break fragile things like necks and backs, and to little and you are sliced like bacon. In any case the seats do have a small delay from initiation, so that things like canopy separation ( but there is a penetrator in any case at the top of the seat if this does not work), harness tension and such can occur before the main event.
If you as a pilot did not put the straps on to the leggings on sitting in the cockpit, so you have your legs pulled off the pedals, they will call you stumpy, as the instrument panel will trim you off at the knees. You might have 2 broken ankles, but at least you will be able to walk. You also are lifetime limited as to how many times you can eject and remain a fighter pilot. 2 strikes and your back might not survive no 3.
I know of one pilot who was compressed 10cm in his unanticipated deplaning, though he did almost make it to the runway. He decided to eject after looking up from the instruments, and saw trees above the canopy. Basically landed next to the fireball, and luckily the fire brigade arrived in 2 minutes to put out the river bed and what was left of the plane. His new nickname was the $10M man....... He was also doing an airshow at 300m, and swallowed a vulture.
My friend Gav was also there, and as he was an armourer, and had serviced that seat, he was very glad it did as designed. Biggest worry I had sitting in the seat to work was checking **EVERY** time I was head over the sill that those 2 locking pins were in. I knew a guy who was sitting and the seat fired. That was a closed coffin funeral, and I was part of the honour guard.