The Atom hub brake that came on my Follis-built within-timeline Mel Pinto tandem (which he seems to have copied from the early Santana design) is built for 48 spokes, Atom tandem freewheel, and is paired with Mafac tandem cantilevers like John's Gitane. No bent axle yet.
I ride in steep terrain and have found three brakes necessary for certain short canyon drops. Although I have no complaints about how the Mafac cantis work with the original non-aero Mafac tandem brake levers (with huge reach), it's possible that a better rim brake/lever combination would allow for rim-brake only control in those few spots where I need all three. For us rim brakes work fine, as Jan describes, for the usual canyon decsents that involve 3-7 miles of 7-14 % drop on twisty road. I've found it's not difficult to control speed on such descents without augmenting with the hub brake and the rims don't heat up (we have about 340 lbs. team and bike). Where we need three is for certain short .5-1 mile canyons that are 20% drop and steeper, and require negotiating with traffic a bit at the bottom. Here again the Atom hub works fine for me and seems to provide positive braking power, but I assume it's nothing like the braking power available from disk brakes, about which I have no personal tandem experience.
One thing not mentioned yet (perhaps because it's too obvious) is that hub brakes are typically used as drag brakes, often set to moderate resistance with a bar end at the top of a descent so they scrub off speed all the way. I haven't used the hub brake this way myself but might try it with certain touring loads. The conventional wisdom over at Tandem@hobbs list where there are many knowledgable and experienced listmembers is that disk brakes cannot be used as drag brakes as hubs are meant to be. They strongly recommend that disk brakes only be applied intermittently like rim brakes. An archive search of T@H at the bikelist.org site will yield lots of information about on-topic brakes. A question about on-topic brakes, however, will only get from T@H stern recommendations that you dump your vintage tandem and upgrade to a modern bike.
Happily using an all vintage French tandem in the mountains, Mitch Harris Little Rock Canyon, Utah, USA