This is fun. You can get a long way with simple tools and a lot of the time it makes sense to not over complicate things, but obviously breaks down at a certain point. What about:
State / failures / retries / indempotency / latency / multithreading correctness..
Data sets larger than one machine can hold.
The fact that other humans (maybe not anymore) have to maintain your shell scripts.
This is how the Plan 9 fork 9front does a lot of its major plumbing, rc scripts. Many tools are just scripts that use rc, awk, sed, bc and other common tools originating on Unix. e.g. the ipconfig tool is an rc script. This creates a zero dependency environment as well as shakes out bugs and shortcomings in foundational software which need improvement.