Failures of any type of component can be a hassle and usually not expected!
Have had a chain jam/bend at the north rim of the Grand Canyon; nearest bike shop 300 miles away. Took me about a half hour of to fix it with mini-pliers and small screw driver.
Have broken front der. on 100 mile ride and then rear cable snapped later in the ride; yup, managed to finish.
Have ripped out half dozen rear spokes with unexplained overshift. Fortunately were within driving distance from home. Hitched hiked a ride home while stoker stayed with tandem. Got our car and picked up stoker/tandem.
For the Di-2 setup we carry no spares; keep tabs on battery by doing a voltage check before each ride with voltage meter.
If Di-2 fails totally (like no power/broken circuitry), you are stuck in the last gear you selected.
However you do get a warning/flashing light to tell you if battery is 'low.' If you don't pay attention to that, then front der. quits functioning but back is still good for a few dozen shifts.
Battery strength/life is dependent on how many shifts and even temperature.
Tested our battery up to 140 degrees Fahrenheit and still functioned; that was parked in the sun at mid-day/ summer in Phoenix for several hours.
Don 't know how well it holds up in freezing temps and no plan to test that!
We tend not to overworry about failures . . . deal with things as they happen.
On multi-day tours we used to carry spare brake/der. cable + folding tire + 2 spare tubes.
Oh, we don't carry a cellphone . . .
After 37+ years of tandeming we have survived all sorts of minor-to-major dilemmas.
Just our experience.
Pedal on TWOgether!
Rudy and Kay/zonatandem