For the entire 10 years I was a Mac user, a Japanese professor named Mitsuharu maintained a fork of Emacs with MacOS-specific features. I never came across any explanation for why the Emacs project didn't accept his changes (i.e., why he needed to maintain his own fork) other than some maintainers' wanting to discourage the use of non-Free OSes.

The good news is that this fork was very competently maintained, so I had a good experience in Emacs on OS X.

The 2 or 3 times I tried "vanilla" Emacs (the one published on fsf.org) it had bugs that weren't present in Mitsuharu Emacs. One of those times vanilla Emacs did not work at all: it displayed a window, but the window was very small (i.e., incapable of displaying more than a few characters) and the usual ways of enlarging a windows on OS X had no effect. My (typing blindly) evalling (set-frame-parameter frame 'fullscreen 'maximized) also had no effect.

The homebrew package named "emacs" got you vanilla Emacs. To get Mitsuharu Emacs, you needed to install the homebrew package named "emacs-mac" instead.

Maybe vanilla Emacs got better on OS X during the past 4.5 years. I haven't had access to a Mac during that time.