Well, I looked at a few today but the store owners refused to let me take pictures. The models on the airsoft stuff vary a lot, especially between the electric and gas blowback models. The issue is where you draw lines on what is "not that difficult", but what I can say conclusively is that it is NOT just a matter of swapping and installing parts. There are a lot of ways to skin a cat, and other more cost effective ways to make functional AR lowers.

Most cost effective for clandestine manufacture on any economy of scale is the polymer lowers molded around sheet metal skeletons. Cav-Arms goes with all plastic, but then their plastic is a strong thermoplastic that requires a lot of heat and specialized equipment to inject into the molds. You can make lowers with weaker lowere grade plastic if the stress points are reinforced with metal (sort of like the way "polymer" frame pistols are made).