I think the major issue that scares some people is the ability of a person who is otherwise ineligible to own a firearm to manufacture one at home using a 3D printer. While it's also possible to do this with other home equipment, proper operation of a lathe, press, and other shop tools usually used to manufacture firearms requires a level of skill that a 3D printer does not. Of course, the cost of purchasing a 3D printer plus materials goes way beyond the cost of an ineligible person getting a gun on the black market. The only benefit that I can see is relative anonymity in acquiring a gun in this new manner.
I figured that something like recent events would happen, which is one of the reasons that I downloaded the plans when they first hit the web. I'm eligible to (and in fact do) own guns, and I trust the courts to protect this. But it never hurts to have an out.