I met one Korean War veteran who did not know that he had a 7.62 X 54R bullet in him, till years later the Doctor pointed it out on X-Ray. The veteran had a pain, got an X-Ray, and there it was.
Of all the WW1 and WW2 personnel accounts I have read, full powered FMJ rounds were effective. The lethality of course depended on what was hit, but people hit with the things generally stopped their hostile activities, even if wounded.
I have never read any WW1 or WW2 accounts complaining that contemporary rounds were ineffective. If people were wounded instead of instantly dying, it was considered their good luck.
However, in the transition between the 450 Martini Henry and the 303 British, complaints from the field did surface. The big lead martini henry bullets were considered better man stoppers than the smaller FMJ.
I have a pre WW1 book written by Thompson and Laguardi on wounds. In that book they make the statement that Civil War mini balls caused more severe wounds than the FMJ of that period.
I can believe that. I was shooting my 577 musket at the range while some buds were shooting their .223 and 308 at some bowling pins. The pins were hanging from strings from the target crossbars. The distance was 100 yards. You could not tell from the reaction of the pins when they were hit with .223 FMJ. The pins jiggled a little bit when hit with 308 FMJ. My buds hollered at me to hit one with the musket. I did. My Minie ball swung the bowling pin once around the crossbar. While this is a totally unscientific analysis of stopping power, it still made an impression on all of us. Those big, heavy, chunks of soft lead appear to hit things all out of proportion to their kinetic energy.
