Carbon is used because it's lighter than a steel or aluminum bike. I have hear the same things as you about the stiffness. Maybe it only has to be stiff side to side not up and down? In any case, there are drawbacks. I get the strong sense that this bike is ridiculous and unreasonable, probably a fun concept, but I hope they prove me wrong and deliver $15k of value and it's not just another luxury "thing". :D