I think the danger here is that now a gun that looks like it's made of plastic could actually kill someone. With painted exceptions, formerly, if someone pointed an orange and blue and white plastic and vaguely gun shaped thing at you, your first response is probably that it's a water squirt gun or some other toy.
Then a very real bullet hits you.
Once a few cops get shot with these things, we can probably expect police to draw their very real weapons on a kid pointing a squirt gun - out of fear that it might be a printed gun.
In other words, with printed custom guns, shape and color recognition go right out the window. It's not so much that they can make a gun, it's that they can make a gun we won't recognize until it's too late.
Oddly enough, plastic guns exist. In fact, they've existed for decades. Even more in fact, your standard issue pistol now isn't made out of metal, it's made largely out of plastic. Most in factly, if you see a police officer in America carrying a standard issue pistol, it's very likely to be a plastic gun.
They were made for a while, but were popularized by Austrian Glock pistols. Glock actually had zero firearms experience when they released their first guns. They were a plastics company, so they built the gun with a plastic frame. I remember, vaguely as a child, hearing that there were plastic guns that would be invisible to metal detectors. I didn't find out until later that they were talking about Glocks (there are enough metal parts, like the barrel and firing pin, that it will still set a metal detector off). Even the 3d printed gun needs a nail to act as a firing pin.
The difference is that a Glock is an insanely durable weapon, and they have some insane stress tests online showing them being beaten to Hell and back, then still firing and cycling correctly. The 3d printed gun they have now can fire, but not well. It's an exceptionally poor gun, though they may someday make it worth something. As it stands, though, you can paint any real gun funny colors and make a fake gun look real. I don't think that is the problem.
I think, if anything, people are afraid of them because there's no paper trail. In America, not having a paper trail on a gun can vary from state to state, and some people literally can swap guns without changing paperwork in certain states. Elsewhere, people want to know who has what kind of guns on their property (imagine that!) so the idea of being able to print out a gun without registering, needing a license, needing to pass a mental health assessment, or needing to have your criminal record checked kind of sets people on edge.
Which it probably should, considering the gun is smoothbore and thus also has no identifying rifling marks to examine during a ballistics test. So not only can you fabricate a gun without any regulation, but then it'd be almost impossible to prove that the bullet came from the gun you fired.
Thankfully, for now, it's fairly easy to get hold of a real gun and the 3d printed one is a piece of shit.