We did a video on the CavArms (now sold by GWACS Armory) polymer AR lower awhile back, and found it to be an excellent piece of gear. However, we had a bunch of feedback from people who had bought other types of polymer lowers and with less-than-optimal results.
One key to using different materials is that part design must match the material choice. The CavArms lower was designed from the ground up to be made of polymer, and has a lot more material around its weak points than the standard AR lower. Other polymer lowers out there, like this New Frontier, are dimensional duplicates of aluminum lowers, and thus quite fragile.