Also, this view may not be flavour of the month (and maybe the OP has already made his mind up/followed the herd, so I am wasting my time even mentioning it) but there are plenty of good reasons not to want disc brakes on a touring bike.
Ooh, do tell, I'm about to buy a disk brake frame. Tell me why I shouldn't?....
If you can find a brake manufacturer that owns up to making 'disk brakes' go ahead, but I think you will be buying a 'disc braked frame'...
. As with most things it is a matter of judgement and priorities.
BTW I agree with those that like BB7s, in that I think they are good brakes, not perfect by any means but pick of the bunch when it comes to readily available cable-operated discs. But picking those presupposes that you want disc brakes, and maybe you don't.
Some issues that you may or may not find significant, off the top of my head, in no particular order;
1) Disc brake fittings make frames heavier. (You don't believe me? -ask Zinn and others who have no particular axe to grind)
2) Disc forks are (if they are steel especially) both heavier and stiffer than non-disc forks. Maybe you need a 1-1/8" steel steerer (eg for heavy loads) but for most purposes 1" is plenty. A steel 1-1/8" disc fork is typically between 1/2lb and 1lb heavier than a rim brake fork with a 1" steerer, and usually feels completely unyielding by comparison.
3) Disc wheels are heavier, and the spokes are more likely to break (they see more stress).
4) If you prang a non-disc wheel or need brake blocks, you will be able to pick something up that will work virtually anywhere; not so for discs.
5) Discs often get bent in transit and when the bike is parked. I would advise anyone who is shipping their bike to remove the discs and secure them between the spokes.
6) If you need to replace a spoke you need to remove the brake disc in most cases.
7) Disc wheelsets have a (weaker) dished front wheel and a rear wheel in which the wheel dish cannot easily be varied. By contrast the rim-brake wheelset on my tourer has an undished front and an almost undished rear, that I (basically) couldn't have with disc brakes. Undished wheels are unquestionably stronger than dished wheels; how much? -well about the same strength if a dished wheel is built with a rim that is about 100g heavier, I reckon.
8 ) Discs and pads get contaminated.
9) discs also wear out
10) discs sometimes crack
11) discs are relatively easily overheated; one lass managed (without trying) to turn her discs blue in the south Downs; what they would be like in the alps goodness only knows. Overheating afflicts cautious riders and those heavily loaded (because they tend to drag the brakes all the way down) more than non-cautious/lightly loaded riders.
12) Hydraulic discs boil up and/or leak oil, and/or come on (or off) by themselves at inconvenient times. I have not yet owned a vehicle with hydraulic brakes that is immune to such things.
13) Organic disc pads can wear faster than any known form of brake when used in wet conditions. I have seen a set of brake pads disappear in a single day's riding.
14) Disc pads are usually ~4mm total thickness, with 2.4mm of friction material. Most brake manufacturers recommend that you change the pads out when they are worn about 1.2mm. Inspecting for wear is difficult and fiddly with some calipers.
15) Sintered disc pads work more consistently than Organic ones and last longer, but may wear the disc faster, are not permissible with every disc, are noisy in operation, and may reject more heat into the brake caliper, exacerbating any overheating/boiling issues.
16) Many disc brake calipers are badly designed such that (when worn) the brake pads can simply drop out of the caliper
17) Some pad/disc combinations have zero initial bite when soaking wet. Think like chrome steel rims and rubber brake blocks; like that. For planned braking this isn't a big problem but I packed in using a disc braked bike for commuting, because I was worried someone would step off the pavement in front of me and I wouldn't be able to stop. (This was well before Charlie Allison BTW....)
18) disc systems overheat on long descents; even BB7s can end up with Salvador Dali- style adjuster knobs when the calipers get properly hot.
19) disc pads vary in quality and durability. I have seen an unreasonable number of brake pads (from several manufacturers not just one) where the friction material has come away from the backing (= no brakes)
20) Discs get hot on long descents; look at the (few) disc brakes that are recommended for use as drag brakes on tandems by trustworthy tandem suppliers. They tend to have mechanical discs that are huge, (300-400mm dia)
21) discs get hot on long descents; when disc brakes were a relatively new idea a standard test was proposed, that equated to a descent of a real mountain road that is regularly ridden by cyclists. When no-one could make a disc brake that would pass this test, the disc brake manufacturers arbitrarily made the test half as difficult to pass. Duh...
22) sandwich construction discs can get so hot the middle (aluminium) layer of the sandwich can melt
23) some (fortunately rare) disc calipers are stupidly designed so that the arm stops moving at what appears to be half-travel, or even goes 'click' and stops working entirely.
24) BB7/BB5 use swivelling (CPS) washers to help align the caliper. These often crack, at best letting the caliper flap about uselessly, at worst jamming the disc or leaving you with a brake that doesn't work. This is enough of a problem that a third party has made stainless washers of the right type available.
25) BB5 and BB7 calipers have a fixed pad adjuster that is threaded into the caliper body. In winter use this often seizes up so badly the brake is scrap.
26) I have never owned a disc braked bike on which I did not have multiple issues of the brake rubbing
27) If you have QR wheels then expect the brakes not to centre the same each time the wheel goes back in and/or the wheel to move when the brakes are used
28) If you are buying disc-braked wheels for touring on the choice of properly strong 36h hubs is rather small. Shimano appear to be giving up on them.
29) Under braking the loads on the hub bearings are between x4 and x8 more than normal ('normal' being on a rim braked bike). I have seen several hubs show premature wear in the LH bearing because of this.
30) discs present a nice sharp edge in even a minor prang. You are not allowed to drive a car around with anything as sharp as a bicycle brake disc edge showing at the front.
31) disc brakes get very hot on long descents; folk regularly burn themselves quite badly on hot brake discs, or ruin their luggage, etc etc
32) if you swap disc-braked wheelsets the wheels invariably don't recentre without the brakes rubbing, even with notionally identical hubs and discs fitted.
33) Your chances of borrowing a disc braked wheel and having it fit and work correctly without a lot of faff are pretty small.
Now I'm not pretending that things are universally rosy for rim brakes or drum brakes either by any means, but I have learned to live with their shortcomings. By contrast I'm not sure I could ever learn to live with some of the shortcomings of disc brakes.
Re the reliability of IGHs and drum brakes. These can be incredibly reliable. In my own experience a fault that either leaves you with one brake or (with the IGH) leaves you unable to ride is very uncommon. How uncommon? Well I'd expect to break a frameset several times (and have done) in the interval between such failures.....
cheers