I’ve also had very positive experience with Tennessee Arms poly lower- got it very cheap at instructors conference the newer one sorta “Gen2” been running it on a tractor/ farm work type rifle, swapped back and forth with 5.56/300 uppers. This is its second MI winter; it’s zero today- spends most of its life outside and several hundred rounds later and some bouncing and abuse it’s still in one piece and working. Another lower made of aluminum is on standby if this one gives up and it’s not my Man the Trenches front line battle rifle but I was curious so I tried one and don’t have any complaints. It is of course lighter than Aluminum but not by much maybe oz or 2 or 3 but there would be other ways to cut weight.I've used Tennessee Arms for years for competitions. No issus. They are beefier than aluminum and have brass inserts for the receiver extension, grip screw, etc.
I can see how all plastic ones would be weaker in those areas, but no issues so far with the TN Arms.
For $120 you can get a aero lower and not worry about a thing.
Don’t see a whole lot of advantages of an 80% polymer lower vs aluminum. Yes a few ounces can be shaved but at the cost of durability.I have the KE lower. Finally got it after what felt like a 10 year long pre-order. I've been to the range with it once so obviously no durability reporting but it seems well made. It is very light, mostly due to it being all one piece and there's not even a proper buffer tube. The buffer rides on plastic ribs inside the stock. I'm happy with it. No way I'd consider one of the mil-spec copy polymer lowers.