I have seen an awful lot of Mossies here in SA over the last 35 plus years (even owned a couple), and I have seen a small number of "fractured" beams BUT, without exception the failure has always been the result of either poor engineered mounting of the beam to the hull (a lot of this arose from "home build" boats and/or bad advice), bad "electrolysis" corrosion (particularly around mounting fixtures), and the most common was from sailing with the dolphin striker too loose allowing excessive, continuous flexing of the front beam. All of these “problems can, and in most cases are, simply addressed.
There used to be a problem very early with the hulls, particularly on sloop rigged cats, when the hulls could literally break off inwards at the front beam due to insufficient structure internally to counter the “inward” rig loads, and as the bow broke occasionally the resulting “rescue” could bend/break the front beam.
I have also seen boats that have been sailing continuously for over thirty years with their original beams and still with no hint of “failure”.
I also remember extremely heated arguments in the seventies about whether the class should “allow” hulls to be made out of PLASTIC (fibreglass). The anti argument being that it would totally DECIMATE THE CLASS, and what self respecting cat sailor would want to sail a “Tupperware” boat when he could enjoy a beautiful crafted timber traditionally built craft? The same argument arose as to whether to allow fibreglass rudders instead of timber, for all the same reasons, none of these changes (and a few others) have ever “hurt” the class, some would say that the opposite is the case, but it always seems to be the case in any “association” that as soon as ANY change/improvement is even voiced, there will very vocal detractors who resist any change at any cost. They used to hide behind such adages as “Tradition”, “destruction of the class”, “if it ain’t broke, don’t fix it”, etc, and would never be prepared to look at any change, even if it was “good for the class” and/or it was necessary to ensure the actual “suvival” of the class, If the “no change” advocates had always won their case we would all still be sailing on “square riggers” or to take it back to the earliest analogy, we would be racing astride a solid log of timber holding up animal skins or palm leaves to catch the wind. “Tradition” is fine in its place, but a certain amount of pragmatism is needed to “stay the same” yet still keep up with the changing world around those “Traditions”.