Basically any imperfection in the path of the projectile before it exits the barrel could easily cause over pressurization that would result in a catastrophic failure. This is why most if not all firearms company perform a test fire before shipment. I believe many states require this.
It seems that you have more knowledge and faith in the predictability of the strength of the output from a 3D Printer than I do. The output of different types of printers and the choices in filament obviously could result in large variations in quality. Even the settings chosen for the same printer could cause differences in the strength of the finished product. I don't doubt that many of the prototypes did not blow up on their initial use. But I know of guns whose primary components were made completely of steel which have had catastrophic failures.
I understand the reasoning that you have presented. One advantage that it would seem to be fairly easy to completely destroy a printed firearm. In most situations I disagree that tracing back a recovered more traditional homemade firearm would be more difficult than finding the person who printed a gun. Currently the availability of parts that could be made into a “zip gun” is almost limitless. The availability of materials and 3D printers capable of producing a gun is far less common. I believe this is true both in the United States and abroad.
I do thank you for the informative discussion. Personally, I won't be switching from the handguns that I am licensed to carry anytime soon.
I won’t be switching away from my ‘normally’ manufactured weapons either - but should things get to the point where those are all gone... well, a Liberator still beats a sharp stick.