"Clean power" as used by Everlast seems to be more of a certification, which is why I need their specifics, and it is not listed as a requirement on most of their products. My generator runs my household items just fine, well pump, AV system, etc. However, the UPSes for the PCs do not like it as the frequency is not as stable as line current, which is typical of most generators.
If a welder, any welder can't be powered by a "normal", run of the mill generator which has wider specifications than line current, I will simply forego a welder purchase until I get into my new home with 400 amp. service and shop. I certainly do not need to try to learn to weld on a machine that will be changing its output as the input current changes. Nor do I need to deal with any potential warranty issues due to power input.