Even the latest Genrac generators still just vary the engine RPM to adjust the output frequency, no fancy electronics or VFD boxes here.
Do they adjust for the frequency, voltage output or generator shaft input speed (1800 RPM)? If the generator component starts going south, you might have 1800 RPM and only 55 Hz at 120 volts or 1950 RPM at 60 Hz and 140 volts. Higher end gensets will monitor all three and alarm/shutdown when stable output (120v @ 60 HZ) can't be maintained by simply controlling engine speed. Even higher end units will alarm/compensate and or shutdown to maintain a perfect power output, generally only found in hospital settings.