FF material is quite a bit thinner and won't cut as easily as B50. Also when a B50 is strung up it immediately stretches and you can feel it, FF doesn't.
I prefer FF over B50, not just for speed, but for string durability, lower noise and less string slap.
Not all bows are designed well enough to handle FF. Recurves are especially susceptible to shearing from FF strings (usually due to square corners on nocks and them being too thin and wide). It's not the strings fault, but rather the bow's nock design. It's probably best to check with the bowyer first.
The only thing I don't like about FF is that I can't get a timberhitch knot to hold on FF, it's too slippery.