My 20kw Generac (on NG) has been flawless over the past 3 years. The longest it's run has been around 24 hours, but it handled it with no issues, and we enjoyed AC when it was really hot
In a previous thread here (exact same topic) there was a member who installs generators, and has for many years...he said he's never had any serious problems with Generac units that he's installed (he said it was hundreds of them). My electrician says he does almost all Generac units and essentially has zero problems with them. Last year during the arctic chill he said there were a couple of units he had installed that didn't want to start (like 10*F below zero, or colder) and all that was required was using a gloved hand to essentially choke the engine for startup.
This is why I've been known to beat the dead horse about people doing a search, and then continuing ongoing threads, rather than starting new ones over and over....all of this discussion was covered in the previous thread just a few months ago. It's better to have all the information in one place so good details or informative posts don't get lost to time.