I never minded my Reynolds carbon rim brakes until descending Col du Glandon last month. That wasn’t fun and on future trips to Alps I will bring alloy wheels.
What wasn’t fun about it? Was it wet? I have Reynolds wheels, and find them fine in the dry, but haven’t ridden big mountains on them. I don’t ride them loads in the wet, but when I do I find them OK, provided you have the opportunity to brake early to wipe off the surface layer of water from the rims. Unexpected / emergency braking in the wet would not be great, as others have mentioned - there’s a second or two of delay before anything much happens!