Small caliber, lots of plastic, short lifespan, unreliable.
Most serious 3D printed guns have at least a metal barrel. Often, 3D printed guns are just a lower receiver, that is the part you hold in your hand, the parts that actually fire the bullets (barrel, pin, slide, etc...) are bought off the shelf from real gun manufacturers.
This is a workaround for some laws that considers gun parts to not be a gun. For example, outside of a gun, a barrel is just a metal pipe and can be traded freely. The part that makes a gun a gun is the lower receiver, and you can 3D print that in plastic and still get good performance. In fact, Glock makes this part in plastic and these are some of the most popular and proven guns in the world.
I think this is why 3D printed guns have not been an issue in Australia. Since you couldn't easily buy all the metal parts you still need.
They aren't really an issue anywhere because if someone really wants a legit gun they are not that hard to buy on the blackmarket or even produce yourself. There just isn't much actual demand for 3d printed guns, people do it because its fun and an engineering challenge. If someone can buy a tabletop mill or lathe, or if they are patient with a file, we have 100+ years of well documented and engineered gun designs anyone can copy. Like you are seriously only cutting a couple hundred bucks off the price between buying a 3d printer or a CNC mill and all the CAD files you need for either version are easily accessible online.
Yeah, and while 3D printing a gun is cool and all, you can’t print bullets, so the person who is interested in printing guns is at the intersection of two hobbies: 3D printing and gun ownership. It’s a niche.
Everybody else who needs a gun for lawful purposes (i.e. self defense) is simply going to purchase one from a reputable manufacturer.