I got early Magpul AR15 mags and had to return them for a recall. I got early Magpul Glock mags and had to return them for a recall. I also have some MFT polymer AR mags and ETS pistol mags.
When they first came out, I was a little leery of polymer mags. My biggest concern was cracking feed lip spread. The old PMAGS used to come with dust covers but I don't think they do any more. The rumor was that it was less of a dust cover and more to relieve pressure from the feed lips to prevent spread. I don't know. I do know that the polymer ones seem to handle some damage better than metal mags. Bent feed lips, big dents, etc. and a metal mag is done. Polymer won't dent or ding or bend if dropped or hit on something. If they do get damaged it's usually a chip or crack and I've seen some in rough shape keep running.
I saw a couple magpul pmags that got shot by a pistol with chunks knocked out of both sides of the mag body and they continued to work fine. I'm not sure but I would guess that a metal mag would have a ridge around the entry hole that would interfere with the spring.