A real problem with bash in particular is that it had many incompatible features which were masked with bash's POSIX mode.

This mode is not set when called as #!/bin/bash but is enabled for #!/bin/sh

The full list of legacy behaviors that are altered in POSIX mode are in a URL at the bottom of the bash manual page:

https://tiswww.case.edu/php/chet/bash/POSIX

Ksh doesn't do this.