Redux is difficult to reason about because side-effects can be triggered from anywhere in the application by middleware.
If you're using something like redux-thunk, a single dispatch can cause both a store mutation AND trigger a side effect, which is confusing. I'd rather have those be 2 separate concerns.
Those dispatches can be triggered from anywhere in the app, and multiple middlewares may trigger multiple side-effects in new and exciting ways.
I find that logic to be difficult to follow and test.
I wish it had a more opinionated way to do side-effects, but the solutions like redux-saga seem even more complicated and confusing.