The thumb injury from a recoiling slide is bad but that may not be the worst part. It could also induce a failure to extract, stovepipe or failure to feed malfunction that would require you to clear the weapon before a follow-up shot. That might be the difference between life and death.
I tried shooting left handed for the first time and didn’t take the time to check the right hand’s thumb position and the slide dug pretty good into the base knuckle of the right thumb. I had it wrapped around the left hand and not laying along side it. I learned...
More difficult with a smashed thumb on the slide operating hand.