Carbon cranks brake. Done it 4 times.
so do metal ones https://www.google.com/search?q=broken+dura+ace+crank&safe=off&source=lnms&tbm=isch&sa=X&ved=0CAgQ_AUoAmoVChMIyPbM-MW9yAIVhJMNCh3KuwJr&biw=1280&bih=636#safe=off&tbm=isch&q=broken+shimano+crank&imgrc=_