A One Wheel requires quite a lot of active balance from the user, a bike requires some balance, but hard cornering on a bike, or slamming the breaks on hard don’t generally result in unplanned dismounts, even with amateur bike riders. A One Wheel on the other hand, even experienced riders will still end up doing unplanned dismounts when performing rapid unexpected manoeuvres (such when avoiding another vehicle or pothole).

It’s only anecdata, but I’ve personally never had a serious unplanned dismount on a bike (after thousands of hours), and includes while mountain biking. I’ve have had a couple of nasty unplanned dismounts on a One Wheel just riding around a park. I suspect broader stats (assuming anyone’s even collected them) would probably find that my experiences of each mode of transport is pretty representative.

You lose drive power on a one wheel and your biology is immediately meeting physics.

Short of the time I was hit by a car, I’ve never had a bicycling emergency — road or mountain — that I wasn’t able to walk away from through some combination of hoping obstacles and/or putting the bike down in a controlled side-skid.