[shrek]

A lot of people, including myself, don't consider macOS 10+ as UNIX because XNU is based on Mach. It does have a BSD wrapper layer and a 4.4BSD-lite derived network stack, but the commonality with FreeBSD in userland code is less than 20% and the kernel code is less than 15%.

Real Unix System V based systems like IRIX, Solaris, AIX, etc. vary wildly in terms of functionality, OS level tools, drivers, file systems etc. Even if they do share a common code base licensed from AT&T way back when, the things that truly make an OS usable/useful in any capacity are completely different from system to system. They're only really united by common application interfaces like POSIX, X11/Motif, and sometimes compilers and other development tools, and so on. OS X is still very much Unix-like and offers pretty decent compatibility with Unix standards and does have certification through the Open Group.