I use both sistems.
On my first touring tandem I have 2 Shimano V-brakes and a drum brake in the front hub. On a step descent, in rain, I cannont slow down the tandem whit all 3 brakes! It was'nt a nice feeling.
On the second touring tandem a use just 2 disc brake BB7 with 203mm rotors. No problem. I ride 4 days in mountains in rainy condition. I can stop the tandem with one finger. Don't forget about rain and mud, this can destroy the rim brakes.
I always ride fully loaded, plus a trailer. More than 200 kilos - we 2 , bike, trailer, paniers.