Instead of saying downsides we can say that there are few compromises of using Redux over Flux. Those are as follows:
redux-immutable-state-invariant, Immutable.js, or instructing your team to write non-mutating code.
If you like this question & answer and want to contribute, then write your question & answer and email to freewebmentor[@]gmail.com. Your question and answer will appear on FreeWebMentor.com and help other developers.