Long before systemd took over the init world, Debian led a huge effort to switch their entire infrastructure to dash[1], specifically making heavy use of checkbashisms[2]. The main reason behind it was performance: POSIX sh implementations were much simpler, and easier to optimise.

[1]: http://gondor.apana.org.au/~herbert/dash/

[2]: https://linux.die.net/man/1/checkbashisms

My rule of thumb is that if a shell script calls for a Bash-only feature, exceeds 100 lines, defines more than three functions, includes a nested loop, or a pipeline that exceeds 80 characters, it's probably time to reach for Python.

There are reasonable cases for exceptions - my backup script has about 200 lines and 6 functions, but it's 100% portable between Linux & macOS.