I do find it amusing that with mechanical systems, especially those you install yourself and have done so for decades, there's a skill set and knowledge that you acquire, which is used semi-frequently by the need to replace cables and re-adjust the system. With Di2 it's install and forget about it as I have with my 3 year old system.
When I started on a ride today, my Garmin would not connect to my Di2 system. I typically have to manually make the BT connection by putting the A Junction into BT discovery mode, it takes a second to do. I pressed today and got zip out of the LED's, so OK, a dead system. I went mt. biking instead and dealt with it after. I then remembered I had removed the bar and stem a few days ago to check the fork, and when I looked, the battery cable had come out of the A Junction. OK, re-plug and good to go, everything shifts fine.. Except no connection to Garmin so BT seems dead. Then I recalled that the BT WU111 system is inline with the battery in the seat tube, so I pulled the seat post, unplugged the battery, waited 5 sec. re-plugged and the BT reset itself and now my Garmin connects. So a different skill set to troubleshoot and one hardly ever used, the system is that reliable.