Would you like to be the person to proof test your shiny new 3-D printed gun?
Reputable firearms manufacturers proof test firearms – fire them with a round that is more powerful than usual – to ensure they will operate safely. This is a dangerous procedure. Reputable manufacturers have safety practices in place for it. Most of the 3-D printing processes produce rough parts; they need
finishing. Do this wrong with your gun and you will have an explosion
when you fire it. (And you may have one anyway, even if you do everything right.)
If you print your own gun you're the tester. It's your hand at risk.
In this bird's view, this whole thing is overblown. It's not hard to make firearms with modern machine tools; hasn't been since the late 19ᵗʰ century. A 3-D printer is just another way of doing that. The hard part is making safe, useful, good-quality firearms through a safe manufacturing process.
On the other hand, the censorship that is imposed on 3-D printed firearms is of concern. Really don't want random officials deciding what designs we can make.