Thanks for your comments, I would use the 90 mm version in addition to rim brakes. I would expect the drum to get very hot. Our old tandem had an Arai, many times so hot I could have cooked eggs on it. However, it never failed and required minimal maintenance over more than 70,000 miles.
Is your SA drum on the front, do you have any steering issues while braking? Does the cable interfere with panniers, racks or fender stays?
I no longer have the bike that had the SA 70 mm drum brakes. It had a front drum brake and a rear 3-speed internal gear/drum brake.
On the front, the main issue I can see is the strength of the forks. A hub brake puts more force on the fork than rim brakes do, for the SA drum this is probably slightly less of a problem than with a disc brake as the reaction arm is quite long so puts the force further up the fork. This reaction arm goes on the inside face of the fork blade, it might interfere with the anchor points for a low-loader front rack. I had no problem with fitting mudguards (fenders).
The reaction arm puts all the braking force on one side. So if the forks aren't strong enough they would either deform temporarily, affecting steering, or permanently, in the worst case breaking either at the fork crown or the point where the reaction arm is fixed to the fork blade. This would be more of a problem with the more powerful 90 mm version and I would not want to fit this to a tandem fork not designed for hub brakes. I believe that a fork designed for disc brakes ought to be OK, but it would be better to get advice from a framebuilder.
I had no problems at all with the 70 mm version, but the bike was used mainly for short rides in an urban setting, although I did test it's performance on (short) steep local hills. I would not have considered using my 70 mm hub brakes for a loaded tourer with long mountain descents.
On the back, the force goes to the left chainstay, which is (usually) triangulated so can resist the braking force more effectively than a front fork. The reaction arm for a rear SA hub brake is shorter, and fits under the chainstay, so no interference with rack or mudguard fittings. A possible issue with mine would have been the braking heat melting the grease for the hub gear internal, but this never happened.
On very long, steep descents, these SA hub brakes might get hot enough to cause either brake fade (drums expanding, brake shoes getting too hot) or melting of the wheel bearing grease leading to damage to the bearings. Due to the design of the SA hubs, IMO the latter problem seems more likely than with an Arai, or even the old Atom rear hub brake I had on the tandem.
SJS sell a replacement for the Arai, with 108 mm diameter drum. But it doesn't look as if it would fit a Rohloff hub.
I also use roller brakes, on the rear on my wife's main bike and my "new" utility bike, where they replace respectively an ineffective long reach calliper and a U-brake (this picked up too much road dirt and then sprayed it onto the chain). I don't like roller brakes as much as rim or drum brakes, the braking action is not so good, they tend to "grab". So I wouldn't like to use them on the front. Another disadvantage of roller brakes is that they use a special grease inside the brake itself, so they need more maintenance than drums.
On the rear, they have the advantage of being compatible with the Nexus 8 Premium hubs. It isn't possible to use a drum brake on these, although I could maybe have used the more expensive solution of Alfine 8 plus disc instead.
The rear roller brake is powerful enough to stop my utility bike and loaded trailer with an all-up weight of about 190 kg max.
Although a recent model roller brake should be quite good at heat dissipation (heavy, with cooling fin) it wouldn't be an option on long steep hills with a loaded tourer or tandem. I tested an older version of a roller brake on an old Moulton about 15 years ago using just the rear brake continuosly on a moderately long steep hill and the grease ran down the spokes onto the wheel rim. New versions would be slightly better, but IMO they would do the same when they got hot enough.
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Maybe the solution for a Rohloff touring tandem would be good rim brakes (cantilever , V-brake or the Magura hydraulics) combined with front
and rear discs ? Twice the braking power of a solo for about twice the weight.
