I have cantilevers (Diacomp 287) and calipers (long reach 105) and find the calipers much better. They have a sharper respones, better modulation and a lighter touch which is essential on a long descent. Calipers limit your tyre clearance to about 28mm which is why we dont use them on tourers.
Regarding disc brakes, these require quite a stiff fork to eliminate bending. Most disc tourers seem to have massive fork blades which are not going to soak up bumps. For sus forks, stiffness is not a problem.
The issue of QRs releasing under disc braking an be eliminated by mounting the caliper on the front of the fork blade. You have to get the disc unit to play ball with front pannier racks and fenders.