Been using all sorts of rim brakes: cantis (Mafacs in the '70s), calipers, U-brake, V-brakes or combinations there of for over 225,000 miles.
Never a stopping issue for us.
Have ridden mountains with long descents (9,200' and d-o-w-n) many times and no brake heat build up issues.
But, we are a rather light duo (135/112 lbs) and we do not just grab a handfull of brake levers when we do a mountain descent.
We do on/off braking front/rear and do not let speed build up too greatly.
Much to be said for braking technique.
Discs can have their own issues including overheating, warping and plastic knobs melting . . . and need to be replaced more often than good rim brake pads.
Have had the old Matthauser brake pads last us 50,000 miles on rear of our tandems and on the front for over 64,000 miles.
Currently using KoolStop Eagle 2 pads on rear of our Zona tandem and Shimano D/A on front.
The rear KoolStops were replaced after 20,000-some miles and the D/As are still originals with over 26,000 miles on them.
Yes, have tried and tested disc brakes on a tandem and felt, that for us, they were overkill.
Just our input/experience.
Pedal on TWOgether!
Rudy and Kay/zonatandem