Carbon posts are almost always going to be more prone to breakage then metal. When you factor in user error in applying too much clamping pressure on post binder on with a work stand, then the number of breakages increase even further. IMO carbon is a great material for a ton of things but for posts and handlebars it is less then ideal. Carbon tubes have poor 'hoop' strength so they are very vulnerable to clamping pressure and/ or pivoting against a clamp; both things which posts and bars are constantly exposed to. You can definitely make carbon work for these applications but they seldom have the same stiffness/ weight/ strength ratio.
I love my Roubaix Comp. and I think it was specd. out great. However, I hated the stem, seatpost, and saddle. I lost 40 grams and gained stiffness by switching to a Thomson X2 stem; lost 40 grams and gained stiffness, ease of adjustment, and safety by switching to an Thomson Elite Setback post; and gained a ton of comfort (for me) switching the saddle out to an Arione.
There has been a bit of talk on here lately about the quality of Specialized posts, seats, etc. I think the Avatar seat was pretty nicely built, it just didn't fit me. The stem was pretty dam heavy and I didn't like the angle options, but otherwise nice enough. The seat post however sucked IMO, it was hard to adjust, heavy, and very flexy. I could feel and see the thing flexing and I just didn't trust it. The Roubaix is plenty damp enough, I don't need carbon in the seatpost or another Zertz insert.