From first-hand experience, I can say that React+Flux has scaled well to 8+ developers over 800+ JS files and ~60k lines of code in a large single page app here at Facebook. I'm happy to answer any questions! Some things that we've struggled with:
1. All data should be kept in stores. You may have some local component state, but it shouldn't be anything you want to persist if the component is unmounted. We have tried using state several times, and always go back to keeping it in singleton stores. It also works better with the action-dispatcher pattern.
2. Now all your data is in stores, but how do you get it into the specific component that needs it? We started with large top level components which pull all the data needed for their children, and pass it down through props. This leads to a lot of cruft and irrelevant code in the intermediate components. What we settled on, for the most part, is components declaring and fetching the data they need themselves, except for some small, more generic components. Since most of our data is fetched asynchronously and cached, we've created mixins that make it easy to declare which data your component needs, and hook the fetching and listening for updates into the lifecycle methods (componentWillMount, etc).
3. Actions don't have callbacks, as they are by design fire-and-forget. So if you need to be notified when some item has finished being created, for example, you need to listen for the follow up action that the CREATE action fires (yeah, actions firing actions, a bit ugly). Even then, how do you know that CREATE_COMPLETED action correlates to the CREATE that you fired, and not another? Well, actions also come with a payload, so what we ended up doing was passing a context object into the payload and plumbing it all the way down into the CREATE_COMPLETED and CREATE_FAILED actions. Being really strict about actions is a major reason why Flux has scaled well for us.