The Di2 battery in my buddy’s road bike discharged over night, and he noticed it only at the race venue. (Yes, he had charged it the day before.) He fortunately found someone with the charging cable, but it was a bit of a hassle.
IMHO these are rare occurrences, but are much easier to mitigate with SRAM. I always keep a coin cell or two in my saddle bag (my power meters have always taken coin cells, as does my heart rate strap and now my two shifters, too). And you can take a spare eTap battery with you, too, they weigh next to nothing and likewise fit into the aforementioned saddle bag.
Personally, I ran out of battery once, because I’m frugal and wanted to use the coin cell battery for a few hours longer. But I had a spare with me, so no biggie. Now I just replace them after seeing the first warning on my Wahoo.
Failures also happen with mechanical drive trains, somehow rust (from sweat) in my Shimano STI levers almost ruined a race for me, because I almost couldn’t shift anymore.