No real need, just wondering (ONAN KY 4000)

I sure like the wave form of the old 18-turn 6.5NH
Volts per Hz, but with a decent governor their reliability is had to beat. Reconnect 30 amperes 240, or 60 amperes 120vac.

Big generators are expensive to convert to inverter. Components like diamonds increase n price to the square of the size.

Because it is "analog", where the wave comes from the crankshaft spinning at 3600 RPM (1800 RPM for older, four pole models), it is a pure analog sine wave in that sense.

Now, how clean it is, especially under load, is a different story. There have been youtube videos testing this, so I'd probably go look there, but Onans are decent, and produce clean power for pretty much anything.

It is pure sine wave and not an inverter type generator

It is not an inverter generator, and is therefore in theory PSW. In practice its waveform isn't nearly as pretty as from a good PSW inverter generator, but it definitely does its job well.