Just a note: one of Flutter's differentiators is that renders every pixel, straight to OpenGL / Vulkan. It doesn't do cross-platform the way RN or Xamarin does it. It's more like Unity.

Since most featured mobile apps nowadays have very custom UIs, we hypothesize that this is a very important feature going forward.

This part I could agree, and for most of my apps make a lot of sense.

I download the code and run the sample where is possible to integrate flutter with the "host" language. Is better than react native, but I wish it have a more ergonomic api.

Most featured mobile apps are still using the native widgets. They're just styling them.

Dart compiles to machine code plus the rendering is mostly impl’d in Dart so you can change basic things about the rendering in Dart, FWIW. In general the dichotomy you’re painting b/w scripting and native languages can hold, but the details make the difference. One interesting thing in this case is having a single language that AOTs but also JITs. The hard thing is re: interfacing to platform APIs, which the OS usually provides in its blessed lang (Java / Obj-C).