Maybe you can get more details on why not? I have a Tailfin X series on order for my Roubaix. I was concerned about stress on the carbon seat post but on the Tailfin site they say it should work fine for aero carbon posts. I am using the through-axle mount since the Roubaix has no pannier mounts.

I don't see how there could be a problem other than Specialized or your dealer not understanding the product and wanting to CYA for any potential warranty claims.