When comparing nuklear vs Flutter, the Slant community recommends nuklear for most people. In the question“What are the best cross-platform GUI toolkits?” nuklear is ranked 7th while Flutter is ranked 15th.
Specs
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Pros

Pro A single-header ANSI C gui library
Pro No dependencies (not even the standard library if not wanted)
Pro MIT/Public Domain License
Pro Focus on portability, efficiency and simplicity
Pro Small codebase (~18kLOC)
Pro Bindings for popular languages
Bindings for languages like C#, Java, Python, Go, Nim, Lua and Rust.
Pro Fully skinnable and customizable
Pro Customizable library modules (you can compile and use only what you need)
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 Very limited documentation
Con No input or output implementation provided
User must connect with OS or other libraries to get input or display output on any platform.
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.
Con Google has a bad history with product loyalty
