I've had the somewhat peculiar experience of braking on a bike when the handlebars weren't completely attached to the frame.

Being able to apply a stopping force without simultaneously being able to fully brace your upper body against a solidly connected part of the bike was a very different, and to my mind, rather unsafe, condition.

I'd think liability issues would pretty grossly limit the merchantability of the powered skateboard. Electric scooters offer the benefit of a handlebar as well.