I'm also curious to hear why you like Flummox better than Reflux. I'm using Reflux now, and haven't hit any pain points, which I can't say the same about some of the other Flux implementations.
When I started investigating isomporphic strategies with Reflux I stumbled upon this thread (https://github.com/spoike/refluxjs/issues/144) which made me realize that if I didn't switch quick I would have some terrible technical debt on my hands in the coming months. I started looking for alternatives and found Flummox, which is lightweight, isomorphic, and written for the future via ES6-7 features like async await and classes. (Once you start using ES6 classes with ES7 classProperties turned on via Babel -> stage 0 you'll never want to go back to the old way of writing things.)