And yet the ECU shipped with nearly every car for the past few decades almost never fails through millions of cycles.

I think all this "ECU code is bad" stuff is just people who don't like the style of the produced code. Are you sure those edge cases aren't accounted for in other ways? Maybe they decided that everything should be "best case" code, and any error just resets the ECU which boots up before the next spark event needs to be triggered.

I just cannot square how much people complain about the code in ECUs with how utterly reliable they are. Consider the group that investigated VW cars cheating on emissions. The system was complex and obfuscated, but also powerful, reliable, and configurable by the manufacturer.

A similar argument was made by Toyota about their systems during the lead-up to the unintended acceleration debacle. I recommend Koopman's talk [1] on the subject for details, but long story short the Toyota ETC was apparently utterly reliable and installed in some of the most popular vehicles on the market at the time. However, detailed investigation of the systems and code behind the few publicized incidents revealed basic system issues like the watchdog failed to detect task crashes, rampant memory safety issues, and even a failure to follow their own internal code guidelines. All of that was manifesting in people's vehicles, but it was only rarely turning lethal and even those cases went mostly unrecognized.

Things have improved significantly since the Toyota acceleration issue, in large part because all those details came out. Model-based design is now basically standard for most ECUs, which eliminates large amounts of buggy human-written code. Formal methods tools that don't entirely suck exist. However, most of the issues Koopman points out (especially memory safety) still exist in many places on modern vehicles and all of his points about the issues with relying on testing to surface quality problems remain relevant.

[1] https://users.ece.cmu.edu/~koopman/pubs/koopman14_toyota_ua_...

https://youtu.be/DKHa7rxkvK8