It's not so much the brakes being stable, it's the operator's body with its c.g. 1m above the board. A maximum deceleration for tires on a good road is generally 0.6-0.8G. So in order to avoid being (literally) thrown off the board the rider needs to be leaning backwards by 30-40 degrees before applying the brakes.
There's no equivalent to this behavior in any other vehicle. In a car you just hit the pedal, on a bike you just squeeze the handles and hold on, on a seque (the closest equivalent I can think of) you must lean back to actuate the stopping.
On a long board at 20mph if you "hit the brakes" you crash, period. That just doesn't sound equivalently safe to me.
In a bike just sqeueezing the handle will also throw you away, especially when downhill. It takes practice as well to be able to move your gravity center backward in emergency brakings.
If you slam on the brakes, yes, you'll fall. However in my experience in practice it is not hard to get used to braking on a longboard. I've done it and seen lots of other people do it without crashing.
You probably don't stop as fast as you would on a car or a bike, but you learn to control the speed you stop at pretty quickly.
I don't think longboarding will ever be as safe as riding a bike, but braking isn't the biggest reason for that.
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.
If I'm going 20mph, "braking" to me means coming to a stop in an equivalent distance as a car or bike going the same speed.
Are the brakes that you've used stable at that rate of deceleration?