Using Information level (rather than the default Warning level that a log file uses) might reveal more.
It seems good, but not perfect. There are known TODO items, according to Real-time backup. More:
You can do your own research, but usually people complain of its slowdown when it DOESN’T run.
USN option working inconsistently talks about soft failures (reported in the log at Information level).
Logging changes to be more specific are also mentioned there. But it’s not clear if this is your case.
Possibly this is just a your-mileage-may-vary situation. Your USN “seems” like it’s doing something.
I am not a USN expert, so I can mainly point to technical discussion from original author and others.