Pulse width modulation works just as well if the pulse width is short compared to the overall cooking time. And not having an inverter makes it more efficient. Unfortunately lots of cheap microwaves have a pulse width which is more than 10 seconds, presumably to help lifetime reliability, but means that for cooking times less than a couple of minutes it doesn't work well.
I once used a very expensive wolf brand microwave that would always run the magnetron for at least 10 seconds regardless of the power level setting. So if you set it to do 10% power for 10 seconds it would actually do 100% power for 10 seconds. I'm kind of shocked that their QA didn't catch this, if nothing else it should refuse to run at reduced power settings for times under ~100 seconds.
