While never having ridden any of these things. It seems to me they have an obvious design fault while riding on a decline, where gravity pulls you forward which leads to the snowball effect of increasing your speed indefinitely.
I have a disclaimer of my own though: I've ridden them on ~flat terrain and only fought these safety mechanism there, trying to go faster. I'd expect them to work just the same going downhill though.
This is what learning balance is all about, and most of us pick it up by the time we're two years old. Segways detect the human riders' effort at balancing (which most do without effort or awareness), and exploit it.