In my experience, the toughest thing to grasp about Flux was how to handle async server actions. It's something that a lot of tutorials (including, unfortunately, this graphic) handwaves, but it's one of the first things you need to nail down if you want to do anything exciting in an SPA.
The todos usually make it seem like you have to to have this flow of information:
ActionCreator -> ApiUtil -> ActionCreator -> Store
...but if you use the same action creator, you end up with a circular dependency. So you actually have to create a separate file of ServerActionCreators, that are only called with ApiUtils:
ViewActionCreator -> ApiUtil -> ServerActionCreator -> Store
This seems like a lot of boilerplate. At my job, we've simplified this a lot by using Reflux, which has async actions that run one Store callback when a call gets initiated, and another related one when it gets completed. But it's not ideal.
Personally, I'd rather see a bigger app than a TodoMVC implementation with a "correct" example of async server actions.