Poly lowers are pretty terrible. There is not enough material around the receiver extension/rear takedown pin for the type of material being used. When they fail, they fail here. The BCG cycling into the receiver extension stresses transfers forces to the portion between the receiver extension and the take down pin and they catastrophically fail.
There is no way to solve this as the design calls for a much higher tensile material in this area. The calvary arms lower seem to be the best in this regard as they added significantly more material/reshaped above the pistol grip around the take down pin/receiver extension necessitating integrating the stock/grip.
A polymer lower might be cool to mess around with in making a lightweight plinking toy but I wouldn't use it for anything serious or on a gun that will see any sort of high round count.