I've used FF on woods as soft as yew and hackberry. I padded the loops up on the yew to 18, but 14 is my normal. Have never had a problem. It's important to make the string grooves a nice teardrop shape to spread the load over more surface area. And it needs to match the string angles perfectly too, so it's not bent around, rather hanging straight from the back of the bow. Have had trouble with wood grain, you need to orient properly or the string may split the wood. Had that happen once, with mesquite and I did read the grain right, it was burl. Only trouble I've ever had, including using elm overlay on a fiarly high performance (after I re-tillered it) Lofton glass longbow, ~60# @ 30" using 14 strand FF no padded loops.
I honestly don't get the precautionary hype about FF, other than for older pre-80s type glass bows. But when you look at the string grooves on those old bows, they are really just notches cut into the glass lams with no re-enforcement or teardrop shape whatsoever. Like duh.