I think the seatpost plays a very large part in this- the rear end of all three of my bikes is alloy, but the Cannondale (stiffest production road frame IN THE WORLD, when new*) is by far the most comfortable- it has a carbon seatpost.
The CX bike with a 31.6mm Thomson alloy post (and wishbone stays) is only tolerable when using much larger tyres- 28c versus the 22c on the Cannondale, and it always surprises me how much less comfortable it is.
I think the seatpost plays a very large part in this- the rear end of all three of my bikes is alloy, but the Cannondale (stiffest production road frame IN THE WORLD, when new*) is by far the most comfortable- it has a carbon seatpost.
The CX bike with a 31.6mm Thomson alloy post (and wishbone stays) is only tolerable when using much larger tyres- 28c versus the 22c on the Cannondale, and it always surprises me how much less comfortable it is.
*Five years ago