Note that Duromax generators are listed at having 12% harmonic distortion. Although it could be a little worse. I'd use 15% as a guide.
For most things, all this means is they will run less efficiently than when using grid power. But for sensitive items (which some electronics are considered), you should charge a battery and then operate the item from the battery (via an inverter). Many UPS units can be used for this, although I've seen some manuals from UPS makers which state a requirement for less than 5% harmonic distortion on the incoming power.
With that said, if it's power for occasional outages, you can usually ignore distortion. However, if it's long term it will take its toll on items which need cleaner power.