I think part of the problem is that we still don't have a standard /bin/sh even though there is a written standard. It feels fair to me to suggest that you can call dash the working standard on Linux, but you can't trust that beyond Linux systems(and I'm ignoring installs that are busybox-based here too).

And that also feels like the problem with extending a script with a few bashisms too, as you're suddenly trying to remember when that feature appeared. Off the top of my head I'm wondering what things won't be available to Mac users because it ships with a pre-GPLv3 bash for example.