FreeBSD is a full-featured OS. Me and my colleagues used it for initial porting of a particular scientific software (non-GUI, just CLI and library) to macOS while having no access to an Apple Mac machine. Almost all the time (modulo Clang version differences), what worked on FreeBSD, also worked on macOS. Even if that was not the case, just using FreeBSD would have helped us a lot to "de-GNU-ify" the assumptions present in the code.

FreeBSD Ports/packages are a great collection of software (~30000); while not always providing the latest version of all software (and I will admit that I am spoiled in that regard by the usage of Arch Linux and derivatives), almost anything you need is there and it just works.