My current Trek carbon is a really stiff bike, either powering uphill or hung out on a bumpy curve. It does not perceptively flex (at least I can't feel it).
Until.......... If you ride it over a set of more-or-less perpendicular train tracks and expect it to rattle your teeth out, it is as soft as a marshmallow. THAT really surprised me. Computer designed frame, taking advantage of the "grain" of carbon fiber for both strength and suppleness. I didn't believe it til I rode one. Makes a Cannondale Aluminum I rode for a week once on a trip feel downright dead by comparison, and Cannondales are really nice bikes.