Are polymer AR-15 lowers resilient enough for .223/7.62x39 recoil?

I don't but a friend of mine does. I personally like my all metal ARs but there is certainly nothing bad to say about the poly AR lower that he has. I built his lower and everything went in easy and is a hell of a lot lighter than my forged lowers.

Not that I see. He's had it for about 6-7 months now and I don't see anything more than a few handling marks.

I have an Omni polymer lower with a steel lower parts kit on my ultralight build. I like it. The whole rifle comes in at a little under 6 1/2 pounds with a loaded 20 round magazine. I don't treat it any differently than my aluminum lowers. I don't feel like it is a fragile piece or that the buffer tube is going to snap off like that famous picture of the Vulcan polymer lower. I'm sure there are more cons that I'm not aware of, but one possibility is that it will throw the balance point of your rifle too far forward. A barrel heavy AR just doesn't work for me. I like most of the weight close to my shoulder; makes offhand shooting easier methinks.

The issue is with those polymer receivers is they look and feel cheap. The weakest point on any AR receiver is going to be the joint where the recoil assembly is located, that is the area that needs to be addressed. As for the one on the market they may have reduced longevity but that's subjective as I cannot find any internet studies.

I have not heard of any problems with them, although I would never get one. I like light ARs, but I think it would be extremely muzzle heavy. My guess is that you will have enough money for an aluminum lower by the time you can find one in stock somewhere to purchase it.