The material used in these 3D printers is very fragile even the metals that can be used in some printers. One shot can be enough to kill, but the duty cycle for one of these contraptions is probably less than 10 rounds. Accuracy is another issue. As detailed as the output can be, precise machining and metal finishing are what makes a gun more efficient and reliable.

I think these things are a potential problem, but years away from being as much a threat as conventional firearms. And stealth relates to metal detectors not Xray or imaging systems.