Say you write an iOS app, and now you want to write the Mac version.
Assuming there’s a data model, maybe a database, some networking code, that kind of thing, then you can use that exact same code in your Mac app, quite likely without any changes whatsoever.
I agree with Brent here. I’ve never really understood the argument that AppKit is that difficult to understand, so that’s why people don’t port native apps over. Surely the underlying logic of the app is the hard part, and linking the functionality to the interface is the easier part?
I would say I’m more of an iOS developer, simply because I’ve spent more time on it. But I’ve also made a few Mac applications. Sure, a resizing window is a bit more complex than a relatively fixed screen size, and some the interface elements are names slightly differently.
It’s just different, for both sets of people. But not as difficult as it may seem.