Worst dev experience 2020:
Working with Xamarin.

Best dev experience 2020:
Working with SwiftUI.

Interesting... how come?

I’n asking because for me working with SwiftUI belongs to the worse development experiences this year, while Xamarin belongs to the better ones (tho still not very good, but far far better than SwiftUI).

SwiftUI was hard to learn, but the more you learn it, the better it goes. I'm at the point now where it just flows for the most part. Missing bits can be replaced by UIKit and there is a good chance that Apple will add the missing parts in a few months. Mix and match with UIKit is very simple.

Xamarin on the other hand...
The more you learn, the more you realize its fundamental flaws, shortcomings and instability.
There is no hope for it to improve because it has been like that for a decade and MS doesn't give a shit about fixing stuff or adding missing stuff. The Xamarin forums are full of complaints about such things and the last words are something like "hope MS will fix that soon" from 2015.
Building UI with xaml is so clunky and boilerplaty and unnatural... I hate it... but that’s a bit subjective.
The way you have to fall back to platform specific stuff in XamarinForms is also terribly verbose and too complicated and limited. And you have to do it A LOT.

Oh, I forgot about the UI previewer. It's so much better in SwiftUI and it allows editing in both directions.
Xamarin is just single direction and crashing all the time.

@100110111
In addition to being built on WPF's awkward foundational concepts, Microsoft moved a lot of key players after they acquired it, mono.

Any time I see Microsoft consolidate, I assume the wheels are about to fall off of something, so the successful bits in the consolidation are being used as donuts to keep the broken thing rolling. Over time those donuts wear out.

@SortOfTested alas that seems to be the trend. Such a shame, though, as Xamarin was so promising early on.

I have to come to a conclusion where my relatively good dev experience with Xamarin is more attributed to the use of F#, that serves to somewhat mask all but the more obvious shortcomings of Xamarin. And, as @Lensflare pointed out, my worse experience with SwiftUI could be attributed to it being quite new to me, and mayhaps I didn’t give it enough time and effort.