I'll make it easy, polyester fibers have a high elongation at break, something in the order of 40-60% depending on the exact fiber. Standard boatbuilding epoxies have an elongation at break of around 5%. Something like G-Flex has 32% elongation, and there are some special epoxies that go even higher. In order to realize the benfits of flexibility you need to at least match the resins elongation to the reinforcement, ideally you want more so that the reinforcement takes all the load and the epoxy remains unfractured thereby maintaining the waterproofing film even if the fibers break.
In reality wood won't move unless it significantly changes moisture content. That's why everyone uses the magic word "encapsulation" that's having epoxy on the entire surface, not only on one side. In order to inhibit the moisture transfer you need to maintain the integrity of the coating. In areas that don't suffer from mechanical intervention a thin coating of epoxy is enough. When it comes to mechanical action you need a thick strong layer. Yes xynole fibers resist abrasion better, but it's inconsequential to waterproofing, the resin will crack long before the fibers give up. Of course xynole uses much more epoxy, so if you want to compare apples to apples you have to use a fiberglass thickness of similar weight per surface area.
But, ultimately it's really simple, on a wood epoxy boat if you see a ding in the paint you start repairing it. If you have areas where you expect frequent mechanical wear you add significant material thickness to soak it up before it reaches the primary wood coating (like for example a skid plate). What you use for it is ultimately a matter of preference there's silicon carbide doped epoxy, there's kevlar felt, polypropylene fibers, etc.
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