It doesn't have to be about the weight/speed. Unless it actually crashes in a fail-safe way, it may keep the blades spinning. It's very unlikely to kill you, but can easily cause pretty deep cuts.

I play with rc planes usually and would really like to see some docs on failure modes / failure rates of typical *copter drones. My planes / controller failed enough times that I'll probably never fly any toy over people. Maybe the newer models are much better. I'd like to know if that's the case.