My experience again, was with frequency switching, not amplitude. I found it to be satisfactory. It was not so much a high frequency generator to my way of thinking, as it was a duty cycle modulation. No power was zero and IAC was at minimum, continuous power application was 100% and IAC was at maximum, switching in between gave intermediate position. As stated, it could go from say 25% to 50% faster than the engine could, so undershoot and overshoot of target RPM was minimized. Amplitude, I think could be affected by resistance changes with temperature or corrosion accumulation on connections and introduce additional errors to be compensated for.
Blacksmith