I simply love Flutter for Android and iOS development and have been using it since alpha. I haven't tried the web application part; I'm pretty sure I never will. Here's my opinion. I've created several apps with Flutter and every time I enjoy it. The UI is easy to make beautiful and the resulting code easy to read. It feels like all what the current Android SDK is missing. The current Android SDK is old and quite frankly painfull to work with. You just can't create beautiful apps with ease; it's always a hassle. It's also messy and not easy to read the resulting code. The way the whole way the framework is structured entails ugly code in my opinion. This all becomes evident when you use different apps on Google Play. You get the sense that every app reinvented their UI; there is no real UI continuity. Google became a freeloader after JetBrains created Kotlin and marketed it will. The thing is Kotlin made just a little better; in terms of boilerplate stuff; the framework I still hate compared to Flutter.
What I really like about flutter is the framework and “it’s all a widget” idea. You can basically do crazy things in UI with ease and on the other hand just use the standard UI components with a great result.
As an example I've created an alternative to Nissan's Connect EV app. It's basically a way to control and monitor your electric vehicle from Nissan. The official app is I’m very disappointed by; it’s slow and full of wrong decisions. My alternative is called "My Leaf" and its available on Google Play and the App Store; https://play.google.com/store/apps/details?id=dk.kjeldsen.ca... https://itunes.apple.com/us/app/my-leaf-for-nissan-ev/id1436...
It's completely open source.