In terms of an FDM printer which most people have, I'd say speed and overhangs are the major drawback to running a business with that type of printer. I do quite a few jobs and my major barrier is some customers wanting either an extremely complex model printed that really should be done on an SLS printer or the average guy that wants you to print a rubber band gun for his kid that he found on Thingiverse. It takes 4 hours to print and thinks he can get it for five bucks. I get lots of those.

Most of my jobs though are folks that want prototypes done and don't balk at the price since it's a lot cheaper to test and redesign before you spend big bucks on molds or machining. Those are the jobs I'm after. I doubt being as small time as I am I could ever make a living at just printing and scanning but I enjoy it and I make a few bucks to play with.