My wife has a 5.56 & 22LR on New Frontier lowers and she rocks & rolls with them, though it's getting tough to afford her ammo habit. If polymer is good enough for Grock, Springfield, et al then its good enough for a black rifle.
If your looking to build an ultralight rifle, ounces count; and the nfa lower is the way to go. You save half a pound over a standard aluminum lower. But I believe that applies only if you use their plastic lower parts kit (plastic hammer, trigger, disconnect, magazine release button, magazine catch, and take down pins). That's a lot of polymer, but the good news is that the polymer trigger group feels great--no creep and breaks clean. With the prices and scarcity of forged aluminum lowers, a polymer lower is not looking too bad right now if you just want something that works.
Have a friend that has 5 of them, only problem is in a slidefire build, you shear the takedown pins, but that is going way above and beyond normal stresses...
sounds like they are worth a shot. found some for 80 dollars online hadnt been able to get my hands on any forged aluminum ones for a while now and the cost of those has dramatically gone. not looking to build a match rifle any way just lite plinker to use when ammo is if ever available again