Yes, if you're using Unity and primarily developing on Windows, most things will 'just work' on Mac. But if you're a big studio with an in-house engine and toolset, supporting a new platform and additional graphics API can be a whole lot of work.
People developing their own frameworks make a lot of simplifying assumptions that turn out not to be true, and then it makes it very difficult to walk them back later. Often people get defensive and try to argue that this is a feature.
Leaving out your deep pocketed customers seems like a pretty dumb move to me.
