When I was in college I worked in a RP lab, and the major concern at the time was build materials. You could build parts out of metal on one of the machines (which could also build parts out of plastic), but you could not build parts out of both materials at the same time.

zCorp parts are extremely fragile, but can be cured hard (but are still more fragile than the other machines), and, at least at the time, it was one of the least accurate machines (but also was much cheaper and used much cheaper materials). The highest quality machines used lasers to either cure resin or melt a powdered form of the material. Both machines and materials were expensive.

In nearly all cases, significant post processing of parts was required to have anything usable when you were done. These are not like printers, where when you're document is done, you hand out copies. When you're part is done you sand. When sanding is done, you cure and/or seal.