Dangerous? Ya think?!? 
I have a 2-speed 29er, and I have no desire to go any faster on one wheel (I’ve cracked the 20mph barrier a couple times). My fastest wipe-out was around 16mph (single-speed 36er).
Here’s a few other reasons:
Worst injuries?
“Worst Pedal Bite” competition!
Broken Leg - With X-rays.
You want speed, get a road bike, or a motorcycle.