TobyGadd

This thread is kind of a disaster, with a mix of conflicting information, unsubstantiated BS, and lots of FUD, obscuring some solid recommendations. But disc brakes were great in 2017, and they're about the same today.

Having ridden an MTB tandem with Magura MT4 brakes, and a road tandem with Avid BB7 brakes, down some steep stuff in Colorado, including Pikes Peak on both bikes, and Mt. Blue Sky (formally known as Mt Evans) on the road tandem, I've learned that disc brakes are reliable and safe. That said, here are some thoughts:

1. Rotor size is important. Both of my tandems have ~200mm rotors. Even bigger might be better, but I would not go smaller based on experiences with my solo MTB.
2. Proper braking technique is crucial. This absolutely means using both brakes so that heat isn't concentrated in just one. Never drag the brakes, but rather slow down quickly before releasing them again to blow off heat (stab and release). It's actually amazing how quickly rotors cool down with exposure to fast-moving air (yes, I did test this). I've tried alternating front and rear brakes, but I'm not really sure that I've seen any benefits--although I also haven't seen any problems either. Either way, dragging is bad, and stabbing and releasing is good.
3. Descend as quickly as you can while still staying in control. Air resistance absorbs an amazing amount of energy, so take advantage of it. Sit up high and stick your elbows out for the most drag. Maybe even put on partially-unzipped jackets before big descents to increase drag. I think that idea of drogue chutes and the like are fun, but I suspect that they would be very dangerous with any crosswinds. My wife and I have joked about adding "webbing" under our arms to add drag.
4. Metallic pads are good, although in fairness I've never tried anything else. Always make sure that you have enough pad material remaining before heading out the door, since metal on metal is terrible.
5. If you have hydro brakes, ensure that they have been bled properly. The only time I've ever brake issues is when there was air in the lines. Also, DOT fluid can absorb moisture over time--which can create vapor in the lines, leading to failure--so fully purge the fluid while bleeding. I've seen MANY poorly-maintained solo mountain bikes with this problem, especially at higher elevations.
6. Keep your rotors clean! If you get oil or grease on them, use lots of brake cleaner--and probably replace the pads. Hell, even braking on the flats with contaminated rotors can be terrible.
7. If in doubt, stop and let things cool down on difficult descents. I've done this a few times when stuck behind slow-moving cars on steep switchbacks when there's really no way to "stab and release" the brakes. If your brakes start chattering, or if you get stressed out, there's no harm in stopping for a bit for the heat to dissipate.
8. I've never tried dragging branches or anything behind me, but that sounds like a recipe for disaster. Ropes getting caught in the spokes/drivetrain? Catching on a guardrail? Bouncing and jerking from side to side? Yikes.

In short, disc brakes are awesome. Even my lowly BB7 brakes, which I figured that I'd have to replace with something more powerful like Paul Klampers, have never let me down. I've never melted or deformed anything on any descents. The only time I've had braking issues is when I haven't properly purged hydro lines or when I've gotten oil on the rotors. Perhaps new tech will be developed that allows people to descend 25% hills for miles while dragging only one greasy brake, but life is pretty good these days.