Your experience as well as mine are what is known as anecdotal. I’ve had zero issues in 2 years (I think it’s 2 years). Not hard to find plenty of folks though that have had problems, usually a battery that has died. How do you deal with that when on a ride ?. Can’t recall, think it stays in whatever gear your are in.

Now let’s recall that the OP stated “1000 mile bikepacking/gravel/offroad journey through California, Nevada and Utah. ​​”. He also stated “I recently found an amazing deal at our local second-hand sports shop. I purchased a complete Shimano road Di2 setup for a ridiculously low price of $200”, so obviously a used group. Probably the single biggest issue folks using Di2 over a few years is going to be a battery dies.

How does the OP deal with that when on gravel roads somewhere in Calif. ?. Would it be a easier to deal with a bent derailer hanger ?, broken brake cable ?. What xtra parts does anybody carry on a long distance off road tour ?, maybe a question for the touring forum, but easy to carry a spare brake and shift cable. Easy to fix mechanical also. Bent derailers are why a lot of tourers have barcons as you can put it in friction.

This all comes down to the particular set of circumstances the OP described. Would I use this system as he described ?, No. Would I use a new system with a known OK or new battery on a cross country credit card tour ?, yes.