There is poor cross-UNIX compatibility if you're doing anything complicated, anyway. I maintained a large test suite for about a year that was written in POSIX

There is poor cross-UNIX compatibility if you're doing anything complicated, anyway. I maintained a large test suite for about a year that was written in POSIX sh and targeted Linux, macos, {Free,Open,Net}BSD. It wasn't fun because every program behaved in slightly different ways, half of them undocumented (for example, I remember having lots of pain with how different versions of tail handled SIGPIPE). In the end it was was easier to rewrite in Perl than to keep maintaining that thing, struggling for hours to find ways of implementing every little bit of functionality that worked reliably on every OS. You'd add or fix something, and the tests would break on FreeBSD. You would fix it there and it would stop working on NetBSD. And so it goes.