I use AltJS for a new project, and frankly, I like it.
2, 4 - This is pretty bizarre because of the “sharing” (if I understand correctly what you meant by that). You create "logical" and "pure (dumb)" components that can later be used many times (it makes no sense to create the same look, but with a different logic, so you just "choose a different store" for the same "pure" "component") . It can also easily track other store conditions, use several stores at once with one component thanks to AltContainer (you should definitely try AltJS with AltContainer) and use actions from one store, while another store is updated (bindListeners).
3 - On the learning curve, I would say that it is quite easy to understand the whole concept. It will literally bring you the maximum of the day to understand the whole idea. Of course, there are more complicated things here, but other libraries also have even more complex things.
5 - You can easily test actions and stores (for example, there is an example on their page) using "AltTestingUtils". You can also mix with Chai or any other claims library that you use. So it’s nice to say
6 - well also was not the idea of Flux about "scalability"? If we think AltJS is probably one of the “cleanest” implementations of this, then I would say that should be good. So far, I can say that it works for me well (although it is difficult to see the future, because now it is not so much). Although there are some tools to improve performance if there are problems with scalability (for example, AltContainer really should be set to ComponentUpdate).
7 - AltJS was made to work in ES6, at least they say it (and I use it too)
While alt examples encourage ES6 and Alt was built with ES6 in mind, it is well suited to use old, old JavaScript.
I made a AltJS post some time ago in a different thread, you might find it useful
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