I've only been at this sport for 15 years or so, but in that time I've been able to shoot a lot of bows, plus make thousands of strings. I've heard rumors about strings damaging bows, but I've never seen a bow made for high-performance materials damaged by the string, never had anyone tell me that a string I made them damaged their bow, never had anyone I know tell me a FF type string caused a failure, or even heard of any definate evidence of such. I've seen a few bows break with dacron strings, and I'm with Pete on this one--what do you blame it on when a bow breaks with a dacron string?
I've been shooing the same bow for 10 years or so and it's never had a dacron string on it. I draw 30.5", and I'm the type that can tear up an anvil with a Q-tip without even trying. It may blow up tomorrow, but it shows none of the signs you warned about. I've used this bow for 99% of my shooting since I've owned it--I couldn't even guess how many thousands of arrows have been through it hunting, practicing, playing, and shooting tournaments.
The only instance I know of where the string could cause more damage being a FF type is during a dry-fire--and that's not good regardless of the string that's on the bow.
I really don't know why a bowyer would charge extra for limbs built to handle a FF string, or shorten the warranty. I've only heard about one that does this (shortens the warranty), and I don't know that for fact, so I'm not in any way trying to target anyone here--I just don't get it. Seems to me the bowyer would want to make the strongest limb he can, regardless. If they don't want their customers to use FF type strings, just don't warranty them at all for those string materials.
I don't claim to know it all, but for my part I need more than an opinion here. Improperly made bows, improperly made strings, dry fires, light arrows, etc. can all cause failures regardless of the string.
FWIW, I don't use these materials (Dynaflight '97 being my favorite) for the extra speed. I like the added durability, reduced stretch/creep, and reduced hand shock. I've only got one small set of cat whisker silencers on my string, and get lots of comments on how quiet it is.
I wasn't shooting bows at the time, but I've heard from many an archer that's been around for a while that the same kind of rumors were floating around when B-50 first hit the market--warnings about that "new fangled" string material blowing up bows, etc. Obviously those folks were mislead.
Finally, every serious tournament archer I know uses one type or another high performance string material. Very few people shoot as many arrows as these guys do. I know Rod Jenkins was (and probably still is) shooting 4 hours a day, pretty much every day. Rod would be one of the first to notice if a bow was damaged by the type string, and I'm sure he'd let it be known if that was the case.
Don't get me wrong here--I appreciate the concern, and understand wanting to warn fellow archers. I just think you have been mislead, based on my own experience.
Chad