I do carry a spare SRAM battery but in 4-5 years have never had to use it.
If I start to get gear noise in certain gears, I know to check RD hanger alignment.
Everything has an expected failure rate. Mechanical shifters can fail, I have had Ultegra fail on me (the little plastic indexer inside). The RD spring on a mechanical derailleur failed. Thus far, no failures to the front or RD or shifter (one 11 speed and two 12 speeds). I have had TT blip switches wear out and get wonky, but they are easy and relatively cheap to replace.
I doubt I would use etap of axs electronic on a touring expedition in a remote area but on a 2-6 week tour? Why not. If something fails, just overnight the part if the LBS doesn't have it and they probably won't