I had a "pleasure" of working with Xamarin Forms and interactions between native UI, behaviour difference between Android manufactures and versions, etc. were a constant source of pain.
With Flutter, if something works on one device it will almost certainly work on all devices. You can also customize per platform.
It is also possible to run it on desktop (although not officially supported). IMO that's a big deal and a potential Electron killer.
Dart is meh though.
Curious that you mention fast, because it is something Electron definitely fails on.
Web apps are for my already installed browser.
QML is as pretty as the design skills of the developer, just like CSS. And better because the full power of the graphics stack is available to the developer, down to pixel perfect graphics, unlike CSS where Houdini still isn't a thing.
Any native application is faster than a Electron based app. It is quite trivial to achieve.