I've only experienced one failure. The claim was the stay should occupy the full extent of the stopover and the stopover (not the stay) should be limited to 4 days. Neither of these breaches were covered in the T&Cs: I kicked back and the reservation was restored.

The notice of the verification failure came through about a week after I made the booking, and a week before the stay. But that hardly establishes a dependable template for action.