- ACID compliant data storage is not something that operating systems excel at, databases are much better. (I've seen some monstrosities where somebody tried to use the filesystem as a database and eventually they always got horrible race conditions and data corruption). This is a table saw; extremely good at one specific thing (rip cuts), can do other types of things in a pinch (cross cuts for table saws, db-as-a-job-queue for databases) and is absolutely rubbish for other things like drilling holes.

- Going up in complexity: Most operating systems I've seen are relatively self-contained on a single box. The OS controls everything on the box and no other OS-es exist on the same server. If you have a large multi-server system the OS typically does not provide too many capabilities except a low level networking stack, something like Erlangs BEAM VM or Kubernetes fills up the gaps to make all the operating systems work together properly. This would be like a CNC machine; large, complex and expensive but very worth it from a certain scale onward.

- Going down in complexity: Embedded systems are often too small for a proper operating system and implement everything they need themselves. Realtime operating systems exist but are often not a good solution for the constraints. This might be something like an impact wrench, super specialized for a specific type of small job.