I dont have any personal experience with the cuben quilt , but considered it.
Only previously made by EE as a stock item I think, others may have provided on on a custom basis.
They are made with impermeable cuben to be moisture resistant.
Basically, they are also mostly airtight, so they wont loft without filling the quilt with air thru a vent tube.
This prevents loss of loft due to body moisture condensing in the fill in cold weather. The only other way to avoid loss of loft on a prolonged trip is thru use of a vapor barrier.
Yes they are clammy obviously if you have to snug up tight in cold conditions.
No reason for temp rating not to be accurate.
They are lighter because of the cuben weight. Just throwing rough #s out here based on memory, but say a 40 F quilt that has ~8oz fill weighs ~ 17oz in 30d (1.1oz/yd) , and 14 oz in 10d (0.65 oz/yd) , might weigh 12.5 oz in 0.51 cuben (0.51 oz/yd) and even less in 0.3 cuben, possibly 11 oz .
It can be a substantial enough difference, to put up with the clamminess for some.