Glancing at the documentation, it seems to be essentially an implementation of the Memento Pattern, as described in the Gang of Four's book. I have to agree, it's fairly straightforward.

I have not used flux. It's not difficult to understand Redux, but the setup is not simple. Defining the constants, defining the actions, then defining the reducers. It's a lot of moving parts. Throw in the connection to react via react-redux, and it feels like you've had to repeat yourself in several places, not exactly, but enough to be annoying.