50 km/h is pretty common descending a hill on a good quality road bicycle. The major difference there is that you will have a properly balanced wheelset (a 700C wheelset alone on a decent roadbike, sans tires, are worth at least $275-350), with gyroscopic stabilizing forces. And you will have brakes. It's also possible on a road bike to aerobrake yourself on a descent by sitting more upright and adding drag.

I go 40 km/h regularly on one of these things, it feels slower than it sounds. I've also fallen at that speed, wearing no protective gear other than a helmet. Not the end of the world, no different than when I fell off a roadbike going roughly the same speed.

If you're worried about scrapes throw on a motorcycle jacket. But if you know how to fall, 40-50km/h is manageable without gear -- aside from the mandatory helmet.

No it isn't. The world record is 146.73 km/h (91.17 mph) by Peter Connolly [0]. While most skateboarders never go that fast, downhill racers routeinly hit very high speeds [1].

[0]: http://www.guinnessworldrecords.com/news/2018/2/watch-this-s...

[1]: https://en.wikipedia.org/wiki/Longboarding#Downhill

Electric unicycle? I own several. Some will even go over 50 km/h, though I don't dare go that fast. It's just safe to have the extra power when needed.