Well, I know that on one tool we saw a 20x or so speedup compared to using Redux. This is because Redux is O(n) in that it has to ask each connected component whether it needs to re-render, whereas we can be O(1).

useReducer is equivalent to useState in that it works on a particular component and all of its descendants, rather than being orthogonal to the React tree.

I think if you can model something with pure functions, you should. That's the approach we try to take for asynchronous processes: just a pure function that you happen to evaluate on some other process. This obviates the need for things like sagas. So I agree with you there I guess.

If you post an example of what you don't think it could handle I will tell you how we would handle it.