For the average joe a poly lower is fine. I have seen all kinds of pictures of these so called "failures". Not one picture was backed up by what happened. I suspect some stupid torture test or plain abuse caused most or all of them. I do not believe one single failure happened from normal operation of firing the weapon. With all that said...Is an aluminum lower better? Yes of course. would I use a poly in combat or competition? No. I have owned one and it was a fine lower with a very good trigger. Great for hunting and plinking. I would not go out and butt stroke crap or drop it in below freezing temps.
I have been watching some 3 gun on TV. It is pretty new to me. I do not see what all the hoopla is about 3 gun being hard on weapons. They pound out a few mags, dump them in a barrel and move to the next stage. Now I have not taken a "carbine course" but they sound harder on weapons than 3 gun. Hell I can dump 500 rounds in my back yard in a day easy.