Magnesium - there is a tendency for people to be a little biased in remembering which boats have troubles. At the moment there are over 100 very fast modern boats, putting massive loads on their structures and doing incredible speeds, all with high tech composite construction. Check out the Jacque Fabre
www.transatjacquesvabre.org/enIt is true that some of these boats have been damaged, but the proportion is much lower than the number of Golden Globe yachts that retired from the last two editions of that race. And these modern things are going about 5 times faster, with correspondingly much higher loadings.
Carbon composite chainplates are much stronger and much better integrated into a boats structure than metal ones fastened with bolts. I hardly ever bolt anything onto my cat's hull, I almost always use modern techniques to make a stronger, watertight, rot resistant and stress reducing composite part that integrates perfectly with the rest of the structure. I think it would be a very silly yacht builder who did not make use of the best available material for their yachts, more and more this will be composite. Cheers
Phil