This is madness. I recently fell off the second rung of a ladder onto concrete (shoe got caught), and even that hurt, third or fourth rung might have been a broken arm.
Falling from this thing is guaranteed serious injury or even death. It would be very easy to add some kind of simple automatic kickstand in case of tipping over laterally.
It's demonstrably not the madness you suggest it is, he's been doing it for decades and he's still in one piece. You see alternative sports athletes walking away from death defying stunts gone awry on a regular basis. Wiping out is a learned skill, and an under-appreciated one. A rough and tumble youth will save you from needing hip replacement surgery when you slip on an icy step when you're 80.
Bikes are even worse generally because you fall laterally and we don't have the neck muscles to stop the head hitting concrete. When you fall off a normal bike it's not normally the speed that determines the head injury.
Falling from this thing is guaranteed serious injury or even death. It would be very easy to add some kind of simple automatic kickstand in case of tipping over laterally.