Professional mountain biker Rebecca Rusch has been on plenty of long rides and experienced enough mechanical failures to know what makes a fully-stocked repair kit. One of her recent journeys, a 1,200-mile journey through the jungles of Vietnam, Laos and Cambodia along the Ho Chi Minh Trail required the utmost degree of preparedness — “We brought with us a lot of extra bearings, o-rings, stuff that you would never be able to find in Laos and Cambodia. Before I go out on these big adventures, I replace every bearing in my bike, my fork got a total rebuild — everything is in pristine condition,” says Rusch.
The fabric attachment system is low-tech, but that seems to be helpful, as it can adapt to seat rails of different widths. One fancier option you might see on more expensive seat bags is a quick-release mount that you install under your saddle. But those have a fixed width, and therefore can fit under seats with only those exact specs. Brooks saddles, for example, are too wide for these mounts.
On the other hand, an on-bike repair kit will be a much smarter selection of tools that you are most likely to need. You should figure out the type of screws that are fitted on your bike and the things that are most likely to go wrong on a ride. Address only these things so as not to burden yourself too much with unnecessary stuff. Moreover, there are bike repair kits used on long rides and those used on shorter rides. The longer the ride, the more tools, and spare parts you will have to bring unless you want to have a very long hike back home.
Modern bottom brackets use a sealed bearing type construction that keeps water and debris locked out, so they seldom go wrong. Maintenance is impossible so they need replacing when they do wear out. Older style cup bottom brackets need to be re-greased every now and again, and occasionally will need to have their bearings replaced. There are a lot more teeth on the chainring cogs than those on the rear cassette, so need changing less regularly. Cranks usually a solid one piece construction so there is not much to go wrong.