Was forced into using it last Christmas to fly to Denver with BA. It was hardly intuitive but eventually I managed to load our "fit to fly" Covid tests. A few days before flying back from Denver to LHR tried to input our return flight but was told this flight has been completed and cannot be accessed.
OK so tried to create a new flight but the app said I couldn't because it will only allow flights originating from LHR.
The check-in staff at Denver took it entirely in their stride and ignored it altogether and acted as though it was a bad joke that management were trying to play on us all.
Don't even start me on trying to get a "fit to fly" test in Colorado Springs.