Most of our docs admit to not having used forceps in years. I've only seen them taken out twice. Once they were just at the rady, but never used. The second time was after an unsuccessful vacuum attempt. Mom was unable to push after a seizure and the FHR was in the 60's. If the forceps hadn't worked, I'm sure we would have ran to OR, as someone had already run to open it. But to be honest, I think by the time we made it to OR, we would have lost mom and baby. In this case, Mom died. It was a true obstetric/medical emergency.
And one of the scariest things I have ever witnessed in L&D.
I think in general, w/o distress, if the doc is not successful with the vacuum, forceps should not be attempted. A CS would be safer.