I read those threads and a lot of the posts were just conjecture not first hand knowledge. Some talked about Plum Crazy lowers which are reported to be made from a different polymer. Before I bought my NF Armory lowers I communicated with several actual owners, most of whom were pleased with their lowers. Some of whom had run them pretty hard and were not experiencing any issues. Do I think today's polymer lowers are stronger than aluminum; probably not. But that does not mean they are not strong enough to get the job done. Like I said before mine are going to be used for dedicated .22LR's but I may match one up to my PSA upper just to see how it performs. Or maybe one of my 9mm upppers.
I will also add that after reading those threads I tested all 3 of my lowers with Pmags, Lancer Warfighters, Troy mags and Brownells aluminum mags and all of them fit, locked and dropped just fine.