Mountain bikers are in different world of repair entirely, one that borders on the comedic absurd. It includes large pumps designed to fill up big, fat tires that squish over things, and a medley of assorted slimes meant to be injected into tubes or tires. In that world, the number of days it takes you to fix your tire and return from the wilderness is a badge of courage—bonus points if you’re bleeding—and we’re guessing that’s not what you’re going for next time you set out for groceries.
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.