They are considered to be cars legally - so you would need a license plate and whats called a vehicle type approval (you would need lights, a seat, steering, two independent brakes and more to get one). Anything faster than 6km/h needs this in Germany. With bikes there is an exception because they only support your peddaling and don’t have a throttle.
When an accident happens you basically driving aan uninsured car and are personally liable for all damages plus the fines for riding an unapproved car in the first place. But on the other hand I have never heard of the police being overly strict - most policemen are apparently intrigued and find the board fascinating.