Sudden deceleration (or acceleration) is probably the last thing you want if the remote disconnects. I'd guess some audible alarm letting you know the remote is no longer functioning + disconnecting the drive system (motors and brakes) so it functions as a regular skateboard would be the safest way to handle that issue.
That might be less than ideal if you were going downhill. Or if you had accelerated to maximum speed, were heading toward an intersection, and planned to hit the brakes before you got there.
I speak from (literally) painful experience. You do not want to have a board decelerating under you, especially when going down hill. If you do not have a decent foot braking technique, you should not ride longboards downhill. Foot braking is (IMO) the most difficult part of longboarding and very easy to screw up, sometimes it's better to try to stay on the board than even attempt braking.
A very slow brake application might be fine, but there has to be an alarm to the rider before that.
Longboards are very dangerous. Usually it's fear and inadequate kicking technique that keeps you from going too fast. I hope that people riding these boards will learn breaking do not rely solely on the engines.
