Polymer AR lowers are problems in waiting. I have seen far too many that cracked, usually around the takedown pins or the threads for the buffer tube.
Yes, Glocks are made of polymer, but their design incorporated that from the beginning, and they still use metal reinforcement in many places. Very few (if any) AR lowers use such reinforcements, and the AR platform was designed for metal receivers.
Yes, plenty of people here will rush to defend them. When you can get aluminum, made to spec lowers for $20-50 more, though, why would you buy the inferior material? That extra $50 (worst case) is less than 10% of the cost of a half-decent AR build, yet it is for the serial numbered component and one of the more critical bts of the rifle.
Never argue with an idiot. They will drag you down to their level and beat you with experience.