I'd say that's a little unfair.
I've been using Pmags since '06 and other then a couple that were part of the recall (that they replaced entirely for free) my Pmags have run better then my USGI mags this entire time.
I know that not a lot of folks on this forum like running their investments hard, but I do. And one of the first casualties of a hard training session in an active gravel pit is magazines. I've had brand new USGI magazines last less then half a day of being dropped , kicked around (moving while shooting and incorperating reloads while doing so, etc.), and just general wear and tear.
The Pmags on the other hand took it all in stride, even when accidentally stepped on. Got a little scratched, but they keep working even when covered in mud. The USGI mags on the other hand....er......not so much. They dent easily when doing combat reloads, don't like being stepped on at all, and i've had more then one decide to **** the bed on me and ust pop the baseplate and dump all my ammo on the ground. Keep in mind i'm talking about nearly brand new Ok mags that had been used maybe 3 to 4 times prior and didn't even have finish wear on them yet.
To put it bluntly, Once I got into training, all of my USGI mags got turned into static range mags because I didn't want to hurt anymore of them. They're fragile.
As to the Pmag/416 issue, Pmags were originally designed to fit Milspec guns. HK 416s were not milspec at the time and technically were not up until the Marines adopted the M27 series of rifle.
All of that being said, having abused several different kinds of magazines in my life in training, my go to mags are Pmags, USGI are static range mags, Thermolds get used as targets, Orlites are used as trade fodder and HK mags get sold for cash to buy more ammo (too many feed lip issues to justify the expense of the HK magazines).
p.s. I paid around 12 dollars apiece for my stash of Pmags (close to 40 of them), so cost vs. USGI mags was never an issue for me.