While not an expert on CF, here are some "answers".
It is much more than "slightly lighter than ripstop", CF can be had in weights such as .5 oz/yd2 or .7oz/yd2 while ripstop bottoms out at 1.1, and that is before waterproofing (CF is already waterproof).
As mentioned, CF is waterproof but not breathable. This is why it is not used as a full enclosure for a down quilt as the lack of breathability makes it all but impossible to compress the insulation within. The non-breathable layer for the UQ makes it a vapor barrier for the user.