Polymer AR-15 lower receivers

Stand alone poly lowers are not suited to any serious use. They break. Simple as that.

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Aluminium can break too, will your aluminium or polymer AR stand up to the Spetsnaz Ak push-up drill?

https://www.youtube.com/watch?v=po_UUm4GeJQ

Recoil transfers from the upper to the lower. If there is a gap between the upper and lower at the buffer tube extension, then the transfer puts stress on lower causing a problem. If it is tight, then it is just a spacer and really should not wear or stress the lower. I would trust the Aluminium over the Poly, but if the Poly fits correctly, it should last for a while.

Probably wouldn't happen in the OP's neighborhood, but I've seen one break when dropped in very cold weather.
If you're mainly looking to save weight, there are ways to reduce mass on an aluminum lower as well.

I have one of the polymer GWACS armory lowers. I also make polymer for a living though primarily for the automotive industry. The GWACS lower is incredibly robust. It's perfectly capable of butt stroking someone to death should you choose to. Mine has a lot of round through it and works perfectly. A bonus is it's lighter than the metal ones. I honestly believe the stock in this one is actually stronger than regular lowers. It's hard to find pics for it but if you find one looking down into the lower you'll understand why. The sides of it are quite a bit thicker than metal receivers.

Do they really hold up under normal conditions?
I am in the middle of my build and came across a polymer lower. The AR will be of carbine length and used for plinking and home defense.
My question is has anybody have any experience with polymer lowers and were they negative or positive.
Forgive me if I posted this in the wrong section.

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Normal use? Possibly.

Heavy use, duty use, competition, HD/SD, I wouldn't count on it.

I have the original Cav. Arms lowers and a GWACS remake. Both of them have held up fine, but I don't do torture tests. I've never felt that they are weaker than my aluminum lowers.

Once again, if it was DESIGNED as a polymer lower, then they work great. If they clone a forged lower in poly, then they can fail pretty hard. Perfect example of what can happen in the linked video. Push ups aside, its not difficult to conjure up other scenario's where similar occurrences could happen in the field.

https://www.full30.com/video/31a73f8fade48648de9f917af92fdd53