When comparing JavaFX vs Flutter, the Slant community recommends Flutter for most people. In the question“What are the best cross-platform GUI toolkits?” Flutter is ranked 15th while JavaFX is ranked 18th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro FX uses hardware acceleration and Java is fast, so good for 2D action games
Also has many built-in effects, like scale, rotate, color shifting, etc. The look and feel is simple to change later, as it uses Cascaded Style Sheets.
Pro Open source
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 A single codebase for iOS AND Android
Pro Fast
The developer's goal is to allow people to make apps running at 120 FPS.
Cons
Con Does not automatically give you a pure platform look and feel
Cannot be used with non-JVM languages.
Con Still in development
According to the website, Flutter is still in its early stages of development.
Con Based on Dart language
Dart is a Java like language, easy to learn and startup fast for millions of Java developers. BUT if you have to learn it ... it's a con.
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.