Read through the overview and the introduction and literally could not tell what Overmind was supposed to be helping with. Everything shown looks like just basic examples of using state normally in JavaScript. What does it actually do ?
Lib like this or Redux seems like to try to better help people who don't have CS back ground to develop. Anyone who had CS training should be able to state management in plain JS or other general purpose programming language.
If you need a general state management library you'd be crazy to write your own. You might be able to do 90% of what the big, popular libraries have done, but that's just settling for something 10% worse for no good reason. Why give up all that knowledge, testing, and creativity when you can build on top of it? Let other people make the mistakes.