A spinning rotor generator may not keep good freq control, and the voltage may wander, but it makes sinewave AC.
Only the very expensive high end brand inverters put out sinewave AC. Most cheap ones are squarewave and the name brand are modified squarewave
https://invertero.com/
If you need to run sensitive electronics, then a GOOD quality generator or a very high end, expensive sinewave inverter will both do the job. Both will cost about the same, you just choose which power source you will use to operate them.
CNG/gasoline/diesel or solar/battery-powered.