The biggest problem I see with these isn't that they're made of Polymer, but rather many of them will be butchered by shade tree machinists when they try to complete them. People who are buying rifles made with these things will be purchasing a crap shoot. It's enough of a risk to purchase a kitchen table build off some mall ninja, who knows barely enough to be dangerous. When you're investing this kind of money in a weapon, you want some type of guarantee as to what you're actually getting, so you'll have some peace of mind as to reliability. This isn't it.
If you're a knowledgeable machinist who has access to an automated machine shop with the proper tooling, fixturing, and programming software, that's one thing. But that is most likely less than 10% of who will end up buying these lowers. Mostly it's people who want to skirt the law in some way, shape, or form.