Personally, I have never been a real fan of the polymer lowers for AR’s.  But one company is making a polymer lower that is Kevlar reinforced, making the polymer lower more resistant to flexing and even breaking under load.  I am not an engineer, but from what I understand about fiber reinforced polymers is that as long as the fibers are oriented properly, it can add strength to the polymers.  It can also help to contain pieces in the event of a catastrophic failure, meaning there is far less polymer shards flying around to harm the shooter and those in the immediate vicinity.

I know there are other Kevlar reinforced lowers out there, and I have not heard of any failures, so there may be some merit to them being a happy medium between true polymer and metallic lowers.  Time and the internet will tell the tale as people buy, complete and build these lowers for use.

Embedded in the post below is a presentation by DATUM Gear on their polymer lowers, as well as a short video of their lower.