Maybe not 3d printed stuff, but 80% kit guns, Polymer80s in particular, seemed to plague the streets while they were in business, probably still do. Having no paper trail beyond the purchase of the kit is definitely an advantage to criminals and the people who provide to them (also criminals).

There’s always going to be a market that these cater to. Even if not serious career criminals, there’s wifebeaters and boozers and such who can’t pass a NICS check and don’t have the connections to get one off the street.