We've got one on a tandem. It came as part of a complete new build from Roberts in summer 2014. Unfortunately due to slightly delayed completion we missed a trip that year but did six weeks from Burgundy through Switzerland, Italian Lakes, Tuscany, Corsica and back to France in 2015. We are probably traveling fairly light, both being reasonably small and our days of camping with the tandem are over, so perhaps not testing it to its limits. First time having a Rohloff and very pleased with it for the tandem. Now arranging another 6 weeks in Sicily, Sardinia, Corsica and back to France starting in April, so no reservations at the moment.
The new tandem replaced one we had had for about 25-30 years which had the standard for a touring tandem of the time of 700C 48 spoke wheels. I was satisfied that going from 700C to 26", no dishing and with better modern rims we would have a wheel at least as strong as before, if not stronger. I suppose the question is whether the 36 spoke Rohloff gives much advantage over the 32 spoke or even whether the additional spokes actually make it weaker due to the additional drillings. Unfortunately I can't answer that but I specified the 36 spoke hub and am confident of its strength. The only real problem we have had with a wheel was on the previous tandem in New Zealand, a broken flange on a Hope tandem hub after less than 1000 miles, and again relatively lightweight, so I suppose problems can happen to any wheel regardless of number of spokes.
Happy tandeming.