There are "few" components that needs platform specific implementations in React Native. But I've never had to create separate projects entirely - just add some build tweaks.

> I create and style all widgets I want from scratch

This is what I hate about React Native. I don't like the custom boilerplate that goes into every new project. But NativeBase kind of solves this problems - to some extent. Using standard React Native components is like using raw HTML without a CSS framework