Touring on a carbon fiber road bike (a 2001 Trek 5200). This generation of Trek CF bikes had steel dropouts with threaded holes for fender mounting, but strong enough to handle a rear rack. I only carried about 20-25 lbs of gear. I did the complete Southern Tier on this bike and my only problem occurred at the end when my Mavic Ksyrium wheel cracked where the spoke attached to the rim; it had no impact on the tour as I didn't notice it until I got home.
I used my road bike because I couldn't afford a dedicated touring bike at that time. Once I confirmed that I loved touring, I bought a used LHT.