Either dependency issues with other gems, or gems that break due to some sort of library in the OS. If you pin all of your versions, and use it in one place, that's less of an issue, but many scripts are designed to have some level of portability (even if it's to a new instance of the server)

In my experience, Bundler has improved a lot with regard to resolving dependency issues over the years. And OS libs are only really depended on by a few gems, no? 99% of them don't use FFI or call OS libs.

Moreover, how often do you really move a script to a completely different OS, where you don't know which OS libs are installed? And wouldn't those missing OS libs also be a problem when writing the script in Bash or any other language?