Recently I was playing with taking a React app someone else made a convert it over to a static site. One might think it would easy to swap out, my first challenge was npm. Pulling down some packages and trying to jam them into the project was a time sink things didn't work right finally I pulled down the react-static project from https://github.com/nozzle/react-static/tree/master/examples and played around with one of the examples. Running the basic example still spammed out warnings, fixing a few things, and I had the sample running, but when I tried to bundle it using the provided scripts the static version in the dist directory was missing essentials. I had given enough time fiddling with it for today, but this was not my experience using frozen flask. It was super simple. It worked as the directions stated with out difficulty. I'm sure I could probably fiddle with react-static some more and get it working, but thankfully I didn't need to in this case. Using Jekyll was equally easy when I built a static site for my Eve online corp. I'm not only one who has had issues with package management, or a library used by another library. Now these tools all have different purposes with a bit of an overlap in that they create websites, having a purpose built tool makes life easier.
If you need components, and are building a large scale project, maybe React is good choice. If you need to convert to a static site after the fact, you might have some work ahead of you. If you build a blog then Jekyll is probably a fine choice. If you want a lot of helpers and custom components perhaps you need an addon. The great thing about development is being able to make a choice and find the right technology for you.