In the case of the Toyota/Denso mess, the code in question had both auto-generated and hand-written elements, including places where the autogenerated code had been modified by hand later. That is the worst place to be, where you no longer have whatever structure and/or guarantees the code gen might provide, but you also don't have the structure and choices that a good SWE team would have to develop that level of complexity by hand.

The toyota code was a case of truly abysmal software development methodology. The resultant code they released was so bad that neither NASA, nor Barr, nor Koopman could successfully decipher. (Although Barr posited that the issue was VERY LIKELY in one of a few places with complex multithreaded interactions).

Which therein lies the clue. They wrote software that was simply unmaintainable. Autogenerated code isnt any better.