The only time I've ever had a problem with live was when the rat's teeth got lodged under scale and when the snake constricted it pushed the teeth into the flesh. Some antibiotic ointment and a shed later and he was right as rain.

I try to get all of mine on F/T, since it's easier/more convenient for me.

I have seen photos of BPs that have been "chewed" on by prey, but don't have any firsthand experience with problems feeding live, so this should be an interesting thread.