Diesel is probably the most reliable but in some locations with natural gas this is used to power stand by systems.
Are you planning on hard wiring it with with contactors so that you isolate your system from the grid and then turn on the standby generator? This needs proper design and engineering and probably installation by a licensed electrician and approval from your power company.
You also have to consider routine maintenance and testing unless your power goes out frequently so your system operates regularly. With diesel you have to worry about the fuel going off so if you are not consuming it you need to either replace it or get it treated. You also have to worry about condensation in the fuwel tank and water build up. You have to periodically run the motor to splash oil around and stop things gumming up. The battery needs to be on a trickle charger if the system is going to sit for months without being run.
Or you can push your luck like people do and hope that it will fire up every once in a blue moon when it is needed even if you totally ignore it for the rest of the time.
An open mind is a virtue...so long as all the common sense has not leaked out.