Basically the issue is that the only control available is turning the wheel, but an unstable scooter actually needs two controls: one to balance the board and one to control the speed.
Having a single control means that they are tied and thus that, discounting air resistance, if the user continuously leans forward, then the board has to continuously accelerate up to arbitrary speeds since turning the wheel to balance also results in acceleration.
Furthermore, it seems that they don't even have a motor powerful enough to always be able to balance the user along with air resistance, so if the rider continuously leans forward eventually the motor is no longer powerful enough, the board loses balance and the rider probably falls down and dies.
Not sure how they could even think of selling a design like that or how they are surviving the lawsuits.
The simplest fix seems to be using four wheels instead of one, which also removes the need of active balancing and is a normal proper design. If "leaning to accelerate" is desired instead of a more normal handle with a throttle control, then it should be achievable by adding suspension springs and detecting their extension.
Not sure if it's possible to have a proper design with one wheel; a thing that comes to mind is having a sort of "landing gear" that can come down and make the board stable on demand. An alternative could perhaps be an internal reaction flywheel, but not sure if that works and is feasible.