Having ridden a few Tarmacs and owning an '18 Roubaix, I'd say it's either the tires/pressure as mentioned or something off in the fit, as the Roubaix has a considerable amount of additional compliance which makes it less rough. What in particular are you finding less comfortable? I wouldn't say the Phenom Comp is a particularly weird saddle at all. What saddle is on your Tarmac, and what widths are each? Have you tried swapping saddles? There are a bunch of variables depending on the setup of each bike that would be putting more weight forward or backward or putting you in a less than ideal position on the bike. Did you have a bike fit done and was it matched or very close to your Tarmac? I found the Roubaix to feel much smoother over all varieties of road surface.