I did a group tour several years ago, 16 riders for one week. Only mechanical failures was a front derailleur spring broke and an external bearing bottom bracket failed. The bottom bracket failure was over several days, not a catastrophic stuck-on-side-of-road type of failure. Rider continued to ride it until reaching the bike ship where he had a new unit shipped to.

All of my bikes use square taper except for an old three speed that uses a one piece crank. Most of my square tapers are Campy which has a slightly different taper (ISO) and a different installation tool, but if I had a failure my research suggests that I could get by with a Shimano (JIS) taper in a pinch.

I think that the external bottom brackets use a fairly common sealed bearing unit, but I have only heard this as I do not have any direct experience with that. But if that was true I would carry a couple spare bearing units if I was going on a long trip with an external bottom bracket.

In Iceland, one of my crank arm self-extractors self extracted and was lost. To fit the frame in an S&S case, I need to pull both crank arms. If I do a trip like that again, I will bring a real crank arm remover instead.