What's bad about MacOS

Licence

MacOS is licensed under a proprietary licence. This alone is reason enough to not use MacOS.

The Kernel

The MacOS kernel is something called XNU (X is Not Unix). It is a very bad implementation of a kernel. It basically just squishes FreeBSD and OSFMK into one "Kernel Mode", which is a very heavy (and incredibly non-performant) way of designing and executing a kernel.

The "Universal Binary"

The "Universal Binary" is a bad implementation of old binary formats merged into a "new" OS.