Bow CONSTRUCTION makes a huge difference--as much or more than the material itself. If the string grooves aren't cut at the proper angle, it can cause a failure.
I've yet to see definative proof of "Fast Flight" being "hard" on a bow. I've broken several bows, and seen/heard of several others--all but one had a dacron string on it. The one that didn't was a BBO that had a faulty piece of bamboo on the back (it cracked when the bow was drawn).
My favorite bow has NEVER had a dacron string on it. I've been shooting it for at least 15 years. I draw 30.5", it's pulling 66#, and my arrows generally weigh around 9 gpp. It's been dry fired, dropped, used and abused--and it's just fine. I didn't have to pay any extra to have it built for "FF" strings. The limbs are yew, the riser is cocobolo.
That being said, it's the bowyer's call on how to state their warranty.
Chad