It's only dangerous if the participants don't know what they're doing, like at small, rural airshows with relatively inexperienced pilots.
I once saw a Helio Courier piloted by a private pilot with under 200 hours total time do a demonstration of the airplane at an airshow. Winds were reported at 12 knots gusting to 37 (it was a very blustery day, I know a VERY experienced pilot who later that day groundlooped his biplane in these conditions), but he demonstrated slow flight--RIGHT OVER the crowd. The crowd was awed, but I was really concerned. Helio Couriers can travel at VERY slow airspeeds (i think somewhere around 35-40 knots), and because of the strong headwind this guy was practically hovering, around 100 feet (if that height) in the air directly over the crowd. If the wind had dropped off suddenly like it had been doing all day long, it would have been all over.
I got my head checked--by a jumbo jet