Experienced Factorio Players - Most Misleading Upgrade - Whole-Factory

I’m collecting feedback for a piece I’m writing: what’s the most misleading "upgrade" in Factorio? I’m writing a piece that uses Factorio as part of a larger argument about systems, bottlenecks, and the difference between improving one visible part of a process versus improving the whole operation. So I wanted to ask people who actually play the game a lot: What’s the most misleading "improvement"in Factorio from a whole-factory perspective?