My issue tends to be that themes that try to emulate a different OS can only do it in style, not in function. So while a screenshot might look similar, it won't function in the same way as the OS it's trying to be, which leads to compromises all over the place... and the aforementioned jank.
Gnome, for example, doesn't have a minimize concept. There is an extension to add it, but it's janky and feels weird. No amount of theming is going to change this, when the underlying system wasn't designed around it.
I could live with the jank and functionality if it would stay consistent over time - but you know it's going to be broken at the next GNOME update, and the next version won't just suck more, but the theme will also not quite work anymore and it's like multiple papercuts every time. Nowadays I have xmonad, xterm and emacs and a few gnome apps but I would replace them in a heartbeat if they annoyed me enough.
Breaking UI and functionality for no reason whatsoever, with no option to change it back, often with mandatory updates, is one of my #1 pet peeves with modern software.
in an alternate universe, most software would come with a core part that does all the specific work and the UI would connect to it with a fixed interface, and you'd be able to swap out whatever you want.
For GNOME, the bigger problem in my opinion is no option for a global menubar. Though there are similarities between macOS toolbars and GNOME headerbars, the former isn’t bearing nearly as much of a load as the latter is because Mac toolbars don't need to cover every function an app provides. Less-used/niche functions just don’t get a button and instead live tucked away neatly in one of any number of menus at the top of the screen.
On the other hand in GNOME apps, if a function isn’t used often enough to earn a spot in the app’s headerbar or hamburger menu it just gets tossed, because otherwise the hamburger menu becomes long and unusable. This results in less functional apps that are not as well equipped to keep up with growth in the user’s skill.
Ubuntu tried to use a macOS style menu bar back in the Mir days, but in my experience that didn't quite work. I ended up with some applications using the global menu bar and others using their own, adding two menu bars right next to each other when the window was maximized. For a macOS design feature like that to work, you need every application to use the specific APIs to integrate with it, or it'll just add to the GUI mess.
Ubuntu had the power to override APIs as they wished, a theme like this probably shouldn't do that, so I don't think there will ever be a stable theme that'll add a macOS menu bar.
Gnome, for example, doesn't have a minimize concept. There is an extension to add it, but it's janky and feels weird. No amount of theming is going to change this, when the underlying system wasn't designed around it.