A distinct disadvantage motor gliders have is the motor. What I mean is, you are flying a ballasted ship all the time. When the weather is strong, you do not need the engine, but it doesn't hurt, when the weather is weak, the motor glider pilot is on the ground sooner as flying with a wing loading somewhere between ½ and full ballast is not an advantage. Six of one half a dozen of the other each has their own advantages and disadvantages.. Having owned and flown gliders, Motor gliders, and sustainer gliders generally the pure glider has more flexibility of conditions and can fly longer into the dying day. Also, the motor glider even if they push safety, needs to stop soaring much higher and try to get the smoke pot lit. In Europe ⅔ of the new gliders sold have at least a sustainer so the momentum is heading toward more of the fleet to have an engine.

As for the records motor glider vs. pure glider records, I have no contributions in either direction.