InboundFlightInfo coverage

Hi there

Are there any infos available about the accuracy and coverage on InboundFlightInfo?

Stated in the FlightXML2 Explorer:
The inbound flight cannot be determined with accuracy for some commercial airlines, particularly when the aircraft registration is not reliably known. If the inbound flight cannot be determined, an error will be returned.

But I would need to know how “good” the service indeed is.

Thank you
Andreas