If only the door is fiberglass and not the frame, then the only difference is in attaching the hinge screws to the door (which presumably is already done in a pre-hung door). If the framing is also fiberglass (why?), then you need to be a bit careful about shimming and fastening the frame into the rough opening. Fiberglass should be screwed rather than nailed, and you need to be more careful in supporting it, prying into position, etc.
A quality exterior door is solid core rather than hollow, but metal & fiberglass exterior doors may be hollow but filled with insulation rather than being solid material. That makes a difference in how well the hinge screws "bite" into the door.