While Servo Stabilizer offers a reasonable degree of protection against input voltage fluctuation, they take a few seconds for voltage correction due to the inertia of the Servo Motor. Today's AV equipment responds in milliseconds, and this kind of delay may be fatal in extreme circumstances. Here is a fantastic comparison video of Servo Stabilizer with Static Stabilizer (Servo unit is replaced by Solid State control unit + EMI filter). The video's author used various output methods to demonstrate how each device behaves with a change in input voltage.