True, but it isn't likely unless you're running the "ultra-light" stuff that has no safety margin. The same argument can (and is) made against aluminum, but aluminum frames have proven reliable - PROVIDED that they are designed with a safety margin.

You can buy an ultra-light anything for a bike that is likely to fail catastrophically if overstressed. Carbon fiber and aluminum do fail more radically than steel or titanium, but the failure is caused by overstress - not by the material properties themselves. If you buy parts with a safety margin (as opposed to the lightest parts you can find), I think that the likelihood of catastrophic failure is low, regardless of material.