People love to talk about the failures of certain ammo. It's almost always founded in a certain degree of truth and BS.
A fellow who served in the military of San Salvador and fled to this country was telling me the other night that he had a buddy who'd been hit multiple times over a period of two years with .223/5.56 ammo and had scars all over his body.
One night he had someone pull a .22LR pistol on him. He sneered at the guy, informed him he'd been hit with a lot worse and that he would kill him even if he eventually died from the wound the guy would inflict on him if he fired. He'd been drinking and was in a foul mood and the guy called his bluff and shot him dead with one shot to the chest. He took one step and that was it.
My father grew up in the country and said that people really underestimate the .22LR. People often die while a doctor is trying to follow the path of the small bullet, which can literally end up anywhere. I also know someone who was shot by a .22LR bullet fired from almost a mile away at a range. He felt a sting to his back, walked about two steps and keeled over. He almost died and had a hell of a time recovering.
Much of the bad rap of the 9mm comes from the ball ammo days, when the military rounds failed to stop enemy soldiers. Since then, bullet configurations and designs have changed, and it's become much more effective. On the other hand, a lot of people have been hit with .45 ball ammo and kept right on going.
Some people will turn and flee from a .25 pistol; others will keep coming, even if faced with a .44 magnum. People are different. The biggest problem, in my view, is that most people who fire a handgun in defense really aren't prepared for the possibility that you may have to resort to your fists, or even a knife, afterwards. If you empty a revolver into some guy's chest and he keeps coming after you, you may have to hit the guy with your fist or with your gun.
People were too quick to jump to the .40/10mm when they perceived that the 9mm didn't suit them.
Of course, now it's hard to get 9mm ammo anywhere, which I thought would never happen.