My septic system runs on gravity, completely independent of the utilities. If gravity fails, I have big problems, because my car could then fly right through the roof of my attached garage.

If the water system fails, it would be easier for me to tote water from a nearby stream or other location for flushing than deal with digging holes in my lawn.

To me the big risk of a sewage system is if you have a public system, and it fails and backs up into your house.