@Brooooooklyn this instantiate a Subject every render(first mount & update). It's not performant.
Not sure whether it's better to change code here or open an issue to react for using a callback to return the initial state.

const [state] = setState(() => new BehaviorSubject());

It seems that this pattern has been implemented internally.
https://github.com/facebook/react/blob/master/packages/react-reconciler/src/ReactFiberHooks.js#L337

but I guess the cost of initializing a function every render is quite same as initializing a BehaviorSubject. Not very sure about it, need some more tests.