Not well reasoned. Mostly of the complaints boil down to 'not knowing' whether a given piece of data is mutable or not.

If you're doing FP with immutable data, there is no ambiguity: you never mutate. Doesn't matter which language you're in, nor whether the data-structure is mutable or not (e.g Redux mostly uses normal mutable JS objects, with the spread operator to avoid mutation when returning a new state).