When Should a Backflow Preventer Be Repaired Instead of Replaced?

Has anyone here had a backflow preventer start leaking or fail an annual test after a few years in service? I’m trying to understand when normal servicing is enough and when replacement becomes the better option. Issues with seals, check valves, relief valves, or shutoff components may be repairable if the assembly is otherwise in good condition. I found this guide on backflow repair and maintenance helpful for understanding common problems and the re-testing process after repairs. For those with commercial properties, how well have repairs held up over time?