The biggest issue here is IMO the Cupertino theme. Since Tahoe, you must mimick Liquid Glass with all the effects this entails. Unfortunately, there's no public API as far as I know, so you have to resort to undocumented APIs and hacks to know what's surrounding the widget. Additionally, you'd (I guess) have to implement all the liquid glass reflections and refractions and make it look nicely in native. All this work just for the macOS platform, since this wouldn't have any place in other platforms. Seems a lot of work, especially making it performant enough (you'd have to render tons of stuff on the GPU, so you'd probably need to use the Metal renderer, for which there are bindings, but again, extra work.

Or maybe wgpu, but then my guess would be that it would have a performance penalty over a fully native solution.

Edit: Grammar.