I was doing some upgrades on one of my bigger quads (12" props, 910KV motors, 4Cell battery), and I was watching the current meter climb past 70 Amps as I ran up the throttle. 70A X 16V = 1120 Watts! And I was feeling the wind blow EVERYTHING in the room that weighed less than 1b. - screwdrivers, knives, a cereal bowl that I had been eating Cheerios from. And of course, every paper in the room.
And as I was working with the motors, I realized just how much torque they had. These are not little motors: They are 36mm diameter, with a magnet height of 18mm.
And then I used my optical tach to find out just how fast they were spinning. 6,920 RPM! Nearly 7,000 RPM on a 12" prop. These motors and props could easily cut an arm or leg to the bone, or cut a jugular vein or nearly rip off someone's face.
The all-up weight of this quad is nearly 7 lbs. Imagine if it was at 400' and fell. Not a pretty sight - even if it didn't hit a person. It would put a large dent in someone's car. It might even go partway through someone's roof.
And the 4Cell battery is 5Amp hours (5000mAH at 16V). The battery is a "40C" type, meaning it will output at least 5Amps X 40 = 200 Amps during a short circuit. 200A X 16V = 3200 Watts. That will get the battery (or anything that is carrying the current from it) really hot, really fast. The lithium used in the battery is flammable. You get the idea.
If my quad fell on someone's roof, I might not know exactly where it landed. If it shorted the battery in the crash (fairly likely), then SOMETHING is going to catch fire.
While the little 250mm and smaller "racer" quads are fast, they have a much smaller motor, smaller props and smaller wiring (which will hopefully act as a fuse during a short circuit), so they aren't nearly so dangerous - although I won't state that they are "safe" by any means.
If you fly big, heavy craft like I do. Please keep all of this in mind. A pic of the quad I was working on is below.