It works, but it feels off. It consumes loads of CPU, it's not close to the normal responsiveness of a real application and the whole feel is that of a video game UI, something alien to the rest of the system.
Putting Flutter Web into Electron honestly sounds like it would just produce slower Electron apps. One of the strengths of Flutter is that it can target multiple native systems with no code changes, so if you're going to provide a native download, just make a native executable and be done with it.
The state of Flutter Web is quite sad because it could easily become the de facto standard for web app development for platforms that would otherwise not allow your app (say, an app that wouldn't pass Apple's stringent requirements).
I don't blame the developers using Flutter to make apps for the shitty Flutter (and Flutter Web) performance, Google made the choice to drop all native components and render their own either through OpenGL or through a web canvas, and the usability problems that stem from that choice are Google's problem. Even after a few hours of messing around with Flutter, I could see its potential for app development. I'll still prefer a "real" native app any time, but I accept that not every dev shop has the resources to maintain separate code bases for the different platforms they support.
That's going to apply _again_ here: I really cannot visualize what you mean, beyond the comparison to Electron, which you nail. Flutter is "electron done right" for people like me building cross platform apps, it's a cross platform rendering engine, not a browser engine.
Presumably, this is immensely frustrating in your experience, but I really can't name a web app that uses Flutter - Flutter Web apps are/should be distributed like Electron apps. What's the Flutter Web app that frustrated you, I'm intensely curious, I didn't know people were trying to attempt that..