Ranked in these QuestionsQuestion Ranking
Pro Plugins for popular IDEs
Telerik provides Visual Studio and Sublime Text plugins for the AppBuilder.
Pro Integrates with Apache Cordova
AppBuilder offers a fast way to import Cordova plugins such as the camera.
Pro Great developer tools debug/hotreload/analyser
Pro Based on Dart language
Dart is a Java like language, easy to learn and startup fast for millions of Java developers.
Pro Open source
Pro A single codebase for iOS AND Android
The developer's goal is to allow people to make apps running at 120 FPS.
Con Young project with a small community
AppBuilder is a young project that's been fairly slow to gain popularity. This means the community is rather small, so there is a limited amount of 3rd party learning resources available.
There is a forum, but it's not very active.
Con Still in development
According to the website, Flutter is still in its early stages of development.
Con Dart is unpopular and never gained serious community traction like Kotlin or Java
Con Does not support 32-bit iOS devices
If you plan on targeting iPhone 5, 5C or earlier, you can forget about Flutter.