99% of what you will read about carbon frames online is idle speculation, recycled rumour or outright fantasy by armchair materials scientists. With that in mind, here are are a couple of things that are more or less true:
1. Spontaneous failure is rare with any frame material. Both metal and composite materials are susceptible to fatigue but the mechanisms of the latter are more complex, less predictable and less well understood.
2. In certain types of crash, a carbon frame is more likely to suffer irrepairable damage than one constructed from certain other materials, e.g. steel. In the same way that aircraft are built to withstand stresses imposed by flying, bikes are constructed for riding - not crashing. That said, all materials can be broken and Ti for example is comparitively expensive and difficult to work with meaning that repairs may not be feasible even if they are theoretically possible.