One other point that the OP made was a system for long term, like end of world stuff. I don't think any of these system will provide power for weeks on end. I think we are talking about systems for multiple days to a week or so. If you want an end of world system I think you are sort of stuck with something that provides renewable power, solar, wind, batteries, all that sort of stuff. Whole different thing. Am I wrong?
Think about even a car engine running 24-7 how long would it take to say accumulate 100,000 miles? say 50MPH 100,000/50=2000hours, 2000hours/24=84days 84days/30=2.8 months call it 3 months. So in one year that would be like running your car for 400,000 miles. Not to mention how many oil changes you would do.
Now think about what engines these little gen units have that are basically law mower engines and you mow your grass say one hour once a week for 25 weeks That’s roughly a year wear per day.
You are not going off the grid with one of these.
As a kid I worked at a golf course and they had two 2 cycle Lawn Boy mowers and as the new kid I ran one of them 8 hours a day 5 days a week mowing around trees and ditches and ponds as did another guy. They bought a new one every year and the new kid got the old one and I got the new one. If you did that for two years they moved you up to greens and tees and other things. I told my dad how crappy these Lawn Boys were that they were shot in two years. He sat me down and we did the math and I realized how good they were compared to our Craftsman.
As a side note they would have me mix gas and oil in one gallon glass wine bottles from the clubhouse and I would tote this glass jug on a rope I put thru the ring over my shoulder all day from patch of trees to patch of trees. I wonder what OSHA would say about that today.