The complexity comes from stitching everything together. You choose your router, your flux library, your build tools, whether or not to use JSX, whether or not to write your CSS with Javascript, which fancy new React specific testing and mocking library you need to use, how to organize your project, what best practices you should follow, what gotchas you will encounter because of Reacts relative young age as an OOP library, etc.

You will always have to deal with complexity, it just depends on what kind of complexity you are willing to stomach. Some people prefer to deal with the complexity of stitching things together, other people prefer to have things stitched together for them and deal with the complexity of many abstractions. Both are fine choices, and we will debate endlessly with each other over which approach is the best approach. (hint: neither are).