Sadly, the specter of maintenance is omitted from Factorio, and acts as a limiting factor on automation in the real world...

When all of your factory bits are randomly wearing down, causing speed mismatches and stochastic outright failures, large and complex chains become limited by your ability to run around and diagnose and fix things. Things also tend to fail in novel and unexpected ways; redesigns can help mitigate some failures, while new failures may be introduced. And as reliability improves, you start seeing failures that occur on longer timescales, which more trivial failures would initially mask.