I've read quite a bit and known and talked to quite a few WWII vets and never noticed any complaints about stopping power. About almost everything else, yes, but not stopping power.
GIs do love to complain, and a smart commander doesn't worry about that, he worries when they STOP complaining...
What I have heard (and too often) is complaints about the "stopping power" of the .30 Carbine, in the Korean winter. Specifically, how the carbine would fail to get through the heavy Korean overcoats, and not stop them.
Even heard some stories (almost certainly made up stories) about how the North Koreans would soak their coats in water giving them "ice armor" when they froze. GI's also love to make up crap...
Did some checking years ago, and the only verifiable times the carbines "failed" in stopping power were mostly when GIs were shoot at them at ranges over 100yds, sometimes as much as 300, and/or at night, and when the Koreans didn't fall down, the carbine was blamed for lack of stopping power.
True? maybe, maybe not, but the word was out, and repeated enough that the Army actually did "something about it", ..sort of.
Friend of mine, guy I worked with for years was a Korea vet. He had been a telephone linesman in Korea, and carried an M1 carbine. He loved it, it was reasonablly light, and as he put it, fortunately he personally, never had to shoot anyone with it.
And he told me what the Army did about the carbine's lack of stopping power. They did something he hated. What they did was, (in his case, anyway) when winter started, they took away his carbine and made him carry an M1 Garand. (MUCH HEAVIER!)
Generally speaking in combat against enemies who are not "Victory or Death" types when soldiers get shot, anywhere remotely serious, they stop attacking until they're at least a little "patched up".