Many home printers will use ABS (think LEGOS, but not as smooth and shiny), there are other options available. Many commercial printers print using a different method that uses Nylon powder fused with a laser. This is what Shapeways uses and what each image links to. Commercial 3D printers can also print some metals, although the cost goes up exponentially.
ABS printed parts are close to but not exactly as strong as the injection molded parts that come from GoPro or any compatible mount manufacturer. The Nylon parts are a bit more flexible if you make the walls thin, but stiffen up by adding material. Nylon can also be colored with RIT dye after printing. I have not had any issues with the 3D printed parts I have on my bike, which also includes Garmin mounts as well as camera mounts. Of course as with anything, how a part is designed contributes a lot to the overall quality and strength. That is why I tried to make the mounts direct types, that go directly from Sony to GoPro or Contour to GoPro, instead of using something like a Sony/Contour to Tripod + Tripod to GoPro.
Considering that this thread is full of homebrew amalgamations of parts and adapters, there is more of those I wouldn't trust than any 3D part I have. We have PVC pipe, Velcro straps, hose clamps, clamps made for other devices, epoxy, you name it. I have always believed, especially after years of trying different things, that direct mounts offer better device protection, stability, strength, lower weight and usually smaller profiles than a bunch of adapters strung together to get something to work. If I can make that happen with a 3D printed part, then that will be my first choice.
Just look at my Contour post above. I had to "build" an adapter to thread a 6 mm rod into the bottom of a RAM-B-273SS-M6U then use a RAP-B-201U-A Composite short extension arm attached to a RAM-B-348U-GP1. This ends up with a camera that is canted out to the side so it doesn't stand 12" above the bars. It is also highly prone to shaking no matter how tight you clamp down the extension arm. With my direct to GoPro 3D part I just use a cheap alloy GoPro style top cap and my adapter slid into the t_rail. Much lower profile and less prone to breaking in the event of a crash. Also the part count is lower, just two instead of three (not counting that the arm is two parts), and one 3D part is cheaper than all those RAM mount parts.