I ran into a situation on FileMaker Server 15.0.2 where a backup drive was failing on macOS Sierra 10.12.1. The external USB 3.0 drive was responding but very slowly and hadn't totally failed. A FileMaker Server backup schedule ran for more than 30 days before the drive finally failed so there were no backups for a month. There were no errors reported by FileMaker server until the drive finally failed.
There should be a way to prevent a backup from running for more than a specified period of time to help identify these types of problems. There is already a time limit option on scheduled scripts so it would be a good addition to have the same option for a backup operation. In addition it would good if FileMaker Server would throw an error if a backup schedule was still running at the time that it was scheduled to run again as this would be an indication that either the schedules are too close together or that there is some other problem.