I'm happy with a nice quiet, free running gear train. My needs are not for high speed or high torque -- just kinematic synchroni zing. I really need SMOOTH accurate geometry though. I think the resolutio n of 3D printers will have to come up pretty far before they work for me. Heck, even the highest definitio n video still shows artifacts of the discrete pixels.
If you want to loose sleep, read up on how they design gearboxes for aircraft -- helicopte rs and the V-22 Ospray in particula r. The requireme nts were high power, light weight, and super reliabili ty. Easy!