Carbon rims on a damp morning are fine. Carbon rims on waterlogged roads are potentially dangerous if you don't account for the massively increased stopping distance.
I have been riding full carbon zipp firecrests and enve wheels since they were released. I have ridden then in poor weather including through flood water and in snow, the wheels are not potentially dangerous and the braking distances are not massively increased.
I also ride with a number of people on this forum who I assure you will confirm that I have no issue with stopping behind the people using alloy rim surfaces in any weather condition.
I also race Cat4 and have never encountered an issue stopping behind someone with alloy breaking surfaces.
If I had any doubts what so ever regarding the braking ability of my wheels, I would not be using them. I find them reasuring.
As previously stated not carbon wheels are created equal and cheap ones do not have the research or technology placed into the braking surfaces, zipp invest massively in this area and reviews generally state breaking performance is on par with alloy rimmed wheels.
you can see one such review at the following URL
http://wrenchscience.blogspot.co.uk/2011/11/zipp-303-firecrest-carbon-clinchers.html
And another at the following URL
http://www.competitivecyclist.com/review-wheels/wheels/zipp-808-wheelset_641.html
Its not carbon wheels which are the problem, its poor quality wheels