In the early years it was a self limiting process - the rocket seats did progress back damage till the individual lost their seat qualification.

There are other factors - I did a tour with a USMC pilot who switched to become a controller after ejecting 3 times*. Back wasn't gone, but his poster said no there Squadron commander wanted someone with his luck....

Sea Vixen
 

I was airborne in a gannet off of HMS Ark Royal (I think it was "Ark" I've not got access to my log books!!!!) on the Beira Straights patrol when we heard a Vixen had lost an engine and was loosing fuel. We vectored a Scimitar tanker to join but the asymmetric trust on the Viven made it impossible for a successful plug in. The vixen pilot (A.T.) called tanks dry descending to 10K to eject. The Lookers seat failed and so he (J.S.) tried a manual bale-out. He got half way out but it is surmised his seat pack caught on the top seat handle and he was stuck and became semi inert. A.T. tried rolling and bunting and I believe tried flying left handed to reach into the "coal-hole" and toss out the observers dangling legs. All to no avail and he eventually ran out of hydraulics. The Sea VIxen hit the water in a 90 degree bank and the Tanker pilot called that both had gone in. The rescue helo however found A.T. in the oggin with a damaged ankle and a scratch on his face from his visor. He had ejected as the Vixen rolled and his seat just bounced along the water (like a Flat stone thrown on a lake). Milliseconds either way and he would have been history but he lived and had a successful career as an MTP. (he was an engineer officer prior to becoming a pilot)

None Emergency has to be Bernhard Lynch..

And that was when you got a bang for your buck, none of this "gentle" acceleration up the rail with multiple charges.