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Ok I'll give you my/our take. Perspective first 40 + years of significant riding with a number of years of club racing both road and mountain biking. Tandeming for 7 years and about 18,000+ miles in that time. Do all my own wrenching and building of bikes including some internal DI2 conversions. Team weight about 330 and we ride in mountainous terrain and hilly country the majority of the time with very seldom long days of flats. We have done Mt Ventoux, Arthers Pass in New Zealand and many other European descents and climbs on the tandem. Arthurs Pass had a number of stokers in tears due to the steepness and exposure so it is intimidating. We started with front DuraAce caliper and Avid BB7 rear disc and 203mm rotor. The descent on Mt Ventoux melted the red plastic locking adjusting rings on our BB7 and every other one on the other tandems. The win zips went thru an entire set of pads the one bike with rim brakes had a terrified stoker whom sprayed water from the bottles on the rim all the way down. Bengal brakes had no issues really. This started my quest for better brakes for the tandem. I have now had Bengal (actually a pretty good simple brake), Hy/Rd mechanical hydraulic ( doesn't self adjust for pad wear but good at the beginning of use) my go to rotor is the Hope V2 floating rotor. We melted a Shimano Ice tech rotor in Germany on a 17% descent (as did another team). We put the first generation 10 speed Di2 hydraulic on last year before an Italy trip in the rear only and really liked the power and modulation. When we had a new frame built this year we decided to go full in with the new 11 speed and RS785 brakes and I think they are the best brakes we have used by far although we have not yet had a chance to do a major descent with them. My wife now warns me when she is riding without hands to do something as I can throw her off the bike with these brakes as they are that powerful. They are kind of a pain to bleed with the long hose to the rear caliper but you only do that once. I would be uncomfortable using them on a non thru axle front fork due to the power. We are presently using 180 front rotor and 203mm on the rear on our Calfee tetra. I am again trying the XTR Icetech rotors to use the full system integration but will probably return to the Hope rotors as they cannot melt. BTW we use an alternating braking technique and do not use the discs as drag brakes. We are not afraid to go fast and 50 mph is not out of our comfort zone. I now never have to worry about blowing off a tire due to rim heat from rim brakes. If you ride mostly flat or rolling terrain they are probably over kill. YRMV