Now the old Thermold stuff that was junk NIB.
Good point. Plastic mags have been around sorta forever.
Many of the "bakelite" (it is not bakelite) AK mags still work. Some people get on well with Orlite mags for ARs or Galils. Like those, despite the bad reputation a few people have no issue with Thermolds. Canadians did poorly and managed to shatter some in deep cold, but I have used some back when it was hard to get mags and even did classes with them. My worst experiences were overheating them, so the feed lips got soft, then cartridges could fly out esp when you dump them for reloads. Cool down, even just empty in the shade on a 95° day: all good again, not actually damaged. I still have a half dozen.
At the same time, I used some other mags that no longer exist. Ram Line did a very pretty, smoky translucent AR mag. It cracked at the feed lips just enough to induce stoppages. You can do bad plastic, but you can also do good plastic. Anyone here heard of the rubber first gen mag-pul MagPuls getting all gooey? It happens to other rubber (a bit of electronics hardware I worked on in the early 2000s got black goo on me earlier this year) but not MagPul stuff. I have total faith in their QC.
Don't forget how long plastic mags aside from MagPul have been around. Glock's first mags were all plastic, no reinforcement. 1982 is 41 years ago. They got the reinforcement more because the sides bulged so the mags wouldn't drop free etc. They always worked AFAIK/IIRC.
The AUG has all plastic mags as well, and they are as far as I know perfectly reliable. They are indeed used as an example of an overall design that is overall vastly improved and more reliable than even the best AR-15 mag. The AUG is called the StGw-77 in Austrian service. Yeah, there are mags for that platform that (including developmental) 50 years old. They all work. Beat up, scratched, surplus mags are coveted and always work.
The SIG 550 switched from the steel mags of the previous guns to all plastic. They are entirely reliable as far as I know (I have a SIG rifle, the mags in the US are expensive and hard to find so we'd have heard if there were some you should not buy).