With no designed mounting points, I wouldn't even consider clamps on a carbon frame. I'm not even sure I would use a rack clamped to the seat post. I have a rear rack on my Yamaha Wabash but it's an aluminum frame and there are threaded mounting points designed for a rack.I would think with clamps you can always mount a rack, but I would be cautious with clamps on a carbon frame. I'm hoping to get an aluminum Creo as my LBS said they're coming, though they said that back in August and you'd expect them to be here by now.
I had a rack mounted to my aluminum cross bike as it didn't have any rack mounts and it worked just fine.