I did about 20,000km on Avid BB7s and currently have another 20,000 on TRP Spyres. My friend has about 7,000km on Hayes CX Expert. All are cable actuated discs, all have been problem free. The BB7s didn't squeal when I first had them, but at some point the front one started to squeal and I couldn't get rid of it. Luckily it wasn't all the time but I honestly could not discover the cause. The TRPs use the standard Shimano pads, and I see them constantly in bike shops throughout Mexico, Central, and South America. The Hayes pads are less common but we still see them occasionally. All the riding on the Avids was in Japan, so there was no worry about finding replacement parts.

Most people I believe prefer cable discs to hydraulic for touring with the thought that you could bodge together some kind of fix if you need a replacement cable and can't find the actual part necessary, but I'm sure that people tour on hydraulic with no issues also.

All that being said, I wouldn't have any issue using rim brakes on a touring bike. They work well and are dependable.