The only times I remember having issues with the M4/ AR 15 is at a training course were we were forbidden to clean the rifle. At about 1200 rounds on a the second rainy & muddy day I started to gum up and have failures to extract. At 1400 rounds I locked the bolt back and wiped out the chamber with my gloved finger. It ran fine after that.
The AK is a very durable weapon and I would not feel under gunned if I had to use one. It does not lend its self to markmanship however.
As for contractors using AK's, it's a matter of logistics. The weapons and ammo are already in theater and do not need export licensing.
SF using AK's? I don't know but I imagene it's to blend in. They tend to bring everything they can get their hands on.
I have had problems related to AR aftermarket magazines.
I have seen basket full of problems related to Wolf and S&B ammo.