I have no experience with AR-10 mags but I've done some research and asked some questions like this before. (Still building first stag 10)
I believe it's 2 separate issues.
-pmag feed lips break after a while, especially in the cold.
-some PMAGs (the 25 round one that isn't labelled M118) have pretty limited length and you can't load more than the saami spec 2.800"
There are PMAGs that handle slightly longer cartridges. Also, PMAGs are cheap and readily available (at least when the market is "normal") so it's not too bad if you have to replace one. Lancers seem to be reviewed as superior, but are also much more $$$ and a bit tougher to find