Well the sudden acceleration you felt was the pushback at 15mph. The board accelerates more than usual in a short time in order to lift the nose up. That's why pushback is the raising of the OW's nose. The only way to do that is to accelerate faster than you mean to, so that your base position is tilted back. It's meant to remind you to lean back to slow down. If you continue to ride in spite of the pushback, and don't slow down, that's where the board may or may not be able to continue balancing and accelerating.
This of course depends on weight, the ground, temperature, and the rider's weight shifts on the board that the motor has to even out and compensate for.
The motor slowing down would entail the rider leaning forward and the nose pitching down, which may not be the best solution. Personally, I think an auditory warning as the board nears its limits would be good. Maybe at +/-80% of its ability, leaving some room for balancing incidentals, with a beep to signal that the rider is going at their own risk. Kind of like an EUC.