No hull failures due to stress of boating. Where there have been issues, is water penetrating the core. If an owner is having hull bottom flexing, then that boat had been abused and there is probably water in the core. Boats have been re-cored. It is a very unusual boat which has that problem.
Most of the negative responses to C Dory's is from people who either never owned one, or owned one and never learned how to drive the boat properly.
I have heard of one boat which took a swamping wave over the transom, but there was operator error--and the boat was bailed out, and got home safely. Another boat was caught in the surf, rolled and pounded on the bottom (may have been some rocks). That boat was very badly damaged, but eventually someone totally restored the boat.
The 25 I bought was badly abused. The cockpit deck was broken and holed (also had water intrusion related to leaving out, uncovered several winters in New Jersey). The bow V Berth platform had the tabbing knocked loose from pounding into seas. The bottom of the hull was as stiff as when it was built. The tabbing was easily repaired. A new cockpit floor installed, and we used the boat all over the USA from Florida to Icy Straits AK.
The secret of the boats is getting the bow down when going into seas, using both trim tabs and hydrofoils on the lower unit of the motor (Permatrim). It will throw some water, but you are in a cabin, which is virtually waterproof from waves from the bow. The very low planing speed also helps. But if the boat is slamming--slow down!