Some of the same issues, but not all of them. While rear seat stays are tiny, often irregularly shaped, and not designed to be clamped onto, a carbon seatpost by is larger, uniformly shaped (so when you clamp something on it fits), and is actually designed to have the seatpost clamp tightened onto it. You don't want to just clamp a rack on there - if you overtighten it you could break the seatpost. But the seatpost comes with a torque rating - I've always wondered if you replace the hand tightened bolt with a regular bolt, and used a torque wrench, if you could still connect the rack.

To be fair, if it's only 5 pounds getting an oversized underseat bag sounds a lot easier, though.