A boat that breaks in half in its normal use is under built to the definition. But: these are race boats, light weight wins races. While the engineering exists to accurately predict the strength of the structure, the data base of expected forces is still deficient. There have been a few attempts to instrument racing boats over the years (I have a couple of those research papers) and always there are surprises with how high the forces were, somewhere, that wasn't anticipated. In a cruising boat you simply add structure to include enough safety margin so that it doesn't break. You don't know in what areas that was necessary and where redundant. A race boat designer can't think that way. The answers to the those questions are learned by breaking boats in half.