It makes sense to only use a single UI toolkit. If some of your stuff is cross platform, it's probably easier to write the single-platform stuff on the cross-platform toolkit than it is to have your devs need to know 2 or more toolkits.

So developers have more transferable skills.

I had to make a web app with a friend and I used Flutter which was absolutely amazing. Easier to work in than react native, and now if I want to work on some Linux desktop apps I have the skills to do so.

Yes, see e.g. https://flutter.dev/docs/resources/faq

yes and yes! Flutter generates a binary for desktop apps, although It's Alpha. In order to generate a binary in Dart (say for something that's serverside) you need to download the flutter SDK and run dart2native. It's rather new so there's still lots of room for performance improvements but it looks very promising.