I retired before Afghanistan so can't comment about how the 5.56 performed "when it mattered".
When I joined (Canadian Military) we carried a 7.62 "C1" produced by FN. Yes, it was "heavy" and had a wood stock but I felt comfortable carrying something that I would use for deer or moose to defend me if the need came to that. And those things were just impossible to kill. They went bang no matter what you did to them.
Like most NATO countries we switched to the 5.56. With the first iteration of the "C7" (Colt Canada - based on the Armalight AR-15 with influence from the M16A3) I lost my sense of security.
They jammed often, they would find the only spec of dirt in a square mile and the breech wouldn't close fully. Most of us had a permanent callous on our right lower palm from whacking the breech assist plunger to get the thing to lock up. The magazines were some kind of cheap composite - if they hit the ground you had plastic scraps, rounds and spring going in every direction.
But what got me the most was the fact that the round was considered, at least in hunting circles, too small for anything more than a ground hog or a coyote. But they were "nice to carry" - had to be a good two pounds lighter, carrying more rounds then the 7.62 that they replaced.
Now, talking to other guys I served with that stayed beyond my years, they told me that they got to the second version (C7A2) for the Afghan mission which apparently addressed all of the shortcomings of the original (which was better suited to guarding a cenotaph at a war memorial than protecting one's life).
And while I get all the arguments and science that says the 5.56 is more than sufficient, as a shooter and a hunter I'm just glad that I didn't do any combat time with a gopher round. Maybe it was fine but I would rather trust my life to something a little more than a 22 caliber.
Although, like I stated, I only every used the first generation rifle and the experience wasn't pleasant. They were "man target" accurate at 300 yards and they had "scopes" which was initially a novelty after using iron sights and you certainly "looked like Rambo" carrying it (you know, it IS one of those Black, Scary Rifles).