What about wheel bearings? Front and rear hub balls are different sizes, and not many have sealed cartridge bearings.

Essentially, good habits include cleaning and inspecting a bike and its components regularly to pick up things that may be amiss. You don't have to have intimate bike maintenance/repair knowledge... but being able to identify when something isn't working properly while riding, or seeing it while cleaning, can short circuit problems later on. Personally, I think that if a bike is in tip-top conditon before a tour starts, the rider is careful how and where the bike is ridden, and regular inspection takes place, there should be few concerns about repairs and the need for spare parts.

I mean... I've even read of people starting tours on virtually worn out tyres, then wonder why they have an inordinate number of punctures early on in the trip.