If we eliminate the obvious, the fact that the belts and motors can pinch a finger as well as a heater block operating at 200+c will definitely burn you, we are left with the one unknown. That is of any hazardous fumes and odors that may be emitted by heating the plastics. We not only can’t see these gasses, but most of us don’t have a device at hand to make a measurement. Good 3D Printing plastics are available online, however.