As I recall, the CavArms stocks had very few problems. Probably on par with other forged alloy manufacturers. I don't know enough about other attempts at it: Plum Crazy, Bushmaster, for two others I can think of.

In light of the fact that polymers are being used for limitless high-strength/light weight applications, I don't think you can make a rational blanket argument that polymer is necessarily bad for an AR lower.

I would consider using a polymer lower for an ultra light SBR, if it didn't require the stamp and process.