Pretty easy to find stories of cored and uncored boats that sank. And steel, aluminum, wood, etc. Bad use of a material doesn't make the material bad. In the sailing world, high performance boats are almost exclusively built with cored construction, many of them are subjected to worse conditions than any Bertram will ever see.
Bertram had a huge problem with delamination. Their boats were falling apart at sea. The problem wasn't coring. It was horrible construction.