I got a custom bike built up with steel, mostly for cost considerations and general level of "trust" with steel frames. However, I've also seen a lot of folks on long rides who kit out steel frames with carbon forks and seatposts to save weight and enjoy some of the shock dampening qualities of carbon.
Folks who are seriously considering a custom frame built up for randonneuring are advised to read up on the Bike Quarterly article about what makes for
a good randonneuring bike. Even if you do not intend to ride anything longer than a double, a lot of the advice is applicable and makes sense.
You'll notice, of course, that the least important topic is the specific material for the frame. Frame weight is relatively unimportant over long distances; what is key is reliability, comfort and then speed -- in that order.