We had the M7 in the F-4.
If you pulled the handle at 1+ mach you were dead.
We had a term for that: "Suicide"
Hell, if you pulled the handle at any speed the seat would more than likely kill you.
Sorry. I'm being too negative.
MB-M7 saved a lot of guys.
But if you didn't get it exactly right, it would cripple or kill you.
When the seat "cannon" fired, it would shorten your back by an inch or more.
Stationary, on the ground, the seat could throw you 350-400 feet into the air. If you didn't have your head hard back and locked against the seat back it would tear your head off. If your arms and legs weren't locked against your body, you could suffer multiple broken bones.
Still better than riding the blow torch to the ranch.