It has a browser because one of its features is a browser.
"Browser source is one of the most versatile sources available in OBS. It is, quite literally, a web browser that you can add directly to OBS. This allows you to perform all sorts of custom layout, image, video, and even audio tasks. Anything that you can program to run in a normal browser (within reason, of course), can be added directly to OBS."
Which must have been useful enough as a plugin that it got added to the standard install.
I also use desktop backgrounds smartly in conjunction with multiple spaces.
First, for clarity, the macOS version of OBS Studio is not a "port". It's a native application designed to run on macOS.
The mentioned audio issues are not unique to OBS. Apple does not feel that applications need to directly capture desktop audio, so third-party programs that inject themselves in to the desktop audio using kernel drivers (iirc) and create a loopback device are required to capture desktop audio. We have a guide on how to do that here: https://obsproject.com/forum/resources/505/
Can you be more specific about how "none of the widgets work properly"? Which features are you referring to (outside the aforementioned audio limitations) that are not implemented?
System shortcuts like find don't really make sense in an application like OBS. There's very few things you'd need to run a find operation on. Communicating shortcuts is something we could do better, however. We are working on an undo function that will help here in case anything is accidentally changed, but with the complexity of operations available in OBS this is a bit of a challenge and will take time to get right.
Our mixer levels are using a global audio industry standard for levels, you can see more about that in our guide here: https://obsproject.com/wiki/Understanding-The-Mixer
It's a Qt app that happens to compile and run on macOS.
If you want a few really obvious example of how the UI works wrong, try scrolling any of the lists with a trackpad. It won't bounce properly. It also won't autohide the scrolling indicator correctly. You need to repurpose cmd+F, ehm, ok, but why did you remove cmd+W to close a window entirely?
It get much worse when you get into accessibility. Turn on keyboard navigation in the system (in System Preferences -> Keyboard -> "Use keyboard navigation…"). Now open a dialog box, like, say, the Add Image source sheet. Hit tab. It won't actually move between the widgets properly; it cycles between the radio buttons, text fields, and checkbox, but can't highlight the push buttons. This is catastrophic for people who depend on this functionality.
But yeah, "vanilla" OBS feels like using an ancient version of Gimp or something.