The question is a bit too simple. Different 3D printing processes have different problems. Issues you might have with a resin printer (eg: messiness, adhesion difficulties, expensive feedstocks) don't necessarily apply to FFF printers, which have other problems (lack of soluble support material, clogging, dribbling) or powder-bed printers (abrasion, clumping, contamination). If you narrow your focus to a particular process - or even better, a particular machine - the answers you get will be more coherent.

Andrew Werby
www.computersculpture.com