I don't have much experience with Scala, but I think the two languages are pretty comparable in their respective ecosystems. The biggest difference I'm aware of is that Scala has typeclasses and F# does not.
I don't know if I would say 'well'. For simple GUIs it's OK but, for non-trivial GUIs I would use the approach to write the GUI frontend code in C# and have it call the F# 'backend'. If for no other reason than that the support and documentation for doing GUIs in C# is much better.
How about mobile?
Never tried, but I'm guessing more or less the same story as above. I would probably start by looking into .Net MAUI for that.
And how does it compare to e.g. Scala?
The biggest difference is that Scala is a much bigger and more of a multi-paradigm language. F# feels smaller and more focused and on its ML roots and functional programming. Not saying that Scala is less 'functional' than F#, but Scala supports you writing your code in a much more OOP way if you want. Yes you can (and sometimes have to) do OOP in F#, but it doesn't feel natural.
For mobile we have FuncUI (on top of Avalonia) and Fabulous (on top of Avalonia, Xamarin and Maui). Most of these frameworks use Elm architecture, but some do not. For example I use Oxpecker.Solid which has reactive architecture.
Can't help with Scala comparison, but at least DeepSeek V3 prefers F# for UI https://whatisbetter.ai/result/F%23-vs-Scala-9eaede00c7e4485...