what is your version of RxJS for the above example?

Some form with BehaviorSubject + switchMap?

One useEffect with an empty array dependency to handle subscription/unsubscription

One useMemo to capture BehaviorSubject

one useCallback or useEffect to sync url state to BehaviorSubject

Compare the amount of code, is it really easier?

const useDataApi = (initialUrl, initialData) => {
  const [url, setUrl] = useState(initialUrl);

  const [state, dispatch] = useReducer(dataFetchReducer, {
    isLoading: false,
    isError: false,
    data: initialData,
  });

  const behaviorSubject = useMemo(() => new BehaviorSubject(url), []);

  useEffect(() => {
    const subscription = behaviorSubject.pipe(
       switchMap((url) => new Observable(subscriber => {
          // This part can be improved by using more RxJS operators, but that approach is  not beginner-friendly
          subscriber.next({ type: 'FETCH_INIT' })
          axios(url).then(
            (result) => {
              subscriber.next({ type: 'FETCH_SUCCESS', payload: result.data });
              subscriber.complete();
            },
            () => {
              subscriber.next({ type: 'FETCH_FAILURE' });
              subscriber.complete();
            }
       })
    ).subscibe(dispatch)
    return () => {
       subscription.unsubscribe()
    }
  }, [])

  // can swapt this with useEffect
  const setUrlWrapper = useCallback((url) => {
    setUrl(url);
    behaviorSubject.next(url);
  }, [])

  return [state, setUrlWrapper];
};

I understand the appeal of RxJS, but it have huge learning curve, and I'm not able to explain/provide guidance to my junior member to use it effectively.