For one you can still write buggy software in it and for another there are some limitations that can be potential deal breakers: http://compcert.inria.fr/compce

For one you can still write buggy software in it and for another there are some limitations that can be potential deal breakers: http://compcert.inria.fr/compcert-C.html http://compcert.inria.fr/compcert-C.html. That links spells out some of the limitations.