The stiffness of the CF means that it takes the stress much faster than the glass for a given load. Plus the fiber orientation means it would do that, too. The biax will help spread the load well across the flange and into the bottom skin of the deck. I think.
The 4-5% elongation of the vinylester epoxy matches the glass well. The carbon will break long before that. But that's fine. If I ever get that far, then I will have a springy deck with broken CF, but likely some intact biax for a little while. I suspect that would be coming off a heck of a wave and slamming with a giant BFT on the deck.
The CF isn't really lots stronger than the glass, but lots stiffer, and lighter, so it is very efficient.
I really want to put a layer of biax/vinylester in the hull between the stringers, but I don't have the time or resin. That stuff is so tough, it would be a great as a last line of defense against punching thru the hull. Oh well.