It’s understandable not everyone has the spare time to exercise their stored vehicles, tools, equipment and toys.
If I did it monthly to every engine that gets used seasonally, I’d have an average of at least a dozen items to fire up, year round, every month.
Proper maintenance and the most applicable combination of fuel care from drain/run dry to stabilize the fuel to use fuel that can handle long term storage is the key.
I’ve used Onan built in generators , never owned one or more to develop a personal opinion on their reliability, so cannot comment specifically to those that take proper care and still have issues, but I would bet that a lot of the issues are due to lack of maint/storage technique vs being inherently unreliable.