While Apple could stand to improve some aspects of their UI toolkits (and you're right that private widgets have long been a frustration), I'd say they're in an entirely different plane in existence relative to Microsoft in this regard.

The most solid of their toolkits is far and away UIKit on iOS. It's got almost all of the best parts of AppKit with over a decade of thoughtful tweaks, polish, and QoL improvements. AppKit has become a bit more neglected since iOS stole the title of favorite child from macOS but is still quite solid. Both have an extensive set of highly capable stock widgets and with both, you can write just about anything imaginable pretty easily without importing a single third-party library. Neither locks you into a WYSIWYG editor or hand-editing XML and are pure-code-friendly, particularly since the addition of autolayout anchors.

SwiftUI is finally getting to the point where it's not so green and is becoming a more practical choice. Though it wasn't a headliner, this recent WWDC brought a number of long called-for improvements.

By contrast, WinUI lacks such fundamental basics as a tableview/datagrid, meaning you're going to be rolling your own or importing third-party widgets much more frequently. While you're not as locked into XML layouts and resources as one would be with Android Framework with it, it's not as friendly to pure code. It's also decidedly mobile-esque relative to AppKit (likely owing to its UWP heritage) which might be a problem if the goal is to build a true desktop-class app.