Those things hit people (Quite a few youtube video's where people have been smacked across the face with one of these) and absolutely nothing happens to send them to the hospital.
Actually these gas fired ones can do quite some damage to a person's face, or body for that matter so I would not think of them as harmless. You can find pictures and videos of some of them going through car windshield etc.
A few years back, we had a Funfly for radio controlled aircrafts and part of that event was a radar run, and the fastest of the day was a radio controlled plane weighing I would guess 6 lbs with a wingspan of 68" travelling 78MPH, quite a dangerous weapon if used by the wrong person.
Most electric ones however are quite light and if it hit you in the back, you would feel the thump but probably not get hurt.