My Nexus 8 Premium has been used numerous times to haul loads. Total loads (me180+bike37+cargo&trailer) were probably of 325 pounds. I have used my bike as a taxi and beer getter (both at the same time), and these loads came up at about 400 pounds, but that's just crazy and I would not do it normally. Care must be taken not to go hectic on the wheel and you should not try to do fast starts (is it of any concern in touring anyway). I would also take hill climbing slowly and walk it with extreme loads and hills (both combined).
I have been using the Nexus for about a year now, and it doesn't seem to be too much in pain. I need to regrease the bearings (winter riding), but that's pretty much it. I have done long distance biking with it and I would trust the Nexus over any derailleur I've ever had.
I cannot speak from experience with the SRAM, but I believe max recommended load should be around 100kg (plus some occasional stresses). The technical docs say only this in regards to load:
Tandem: Not suitable for tandems, trademen’s delivery bicycles and similar
Of course, Shimano and SRAM are just trying to keep you from putting too much stress on the system. I would NOT use it everyday under heavy loads, and as I said earlier, you should be gentle on it when you are heavily loaded (250+ pounds).
There is also the SRAM P5 Cargo that is rated by them at 120kg and is accepted by them as a tandem hub.