>Effect systems are good when all you want to do is write code that has effects. Monads are not really about effects, although they can be used to describe certain classes of them. Monads are nevertheless, strictly more powerful.

This is not true. Monads and effect systems are equally powerful.

More concretely: Effect systems are essentially delimited continuations. Monads and multi-shot delimited continuations are equally powerful, though some effect systems don't support invoking the continuation multiple times and so aren't multi-shot and therefore are less powerful than monads.

You can express any monad with delimited continuations, and vice versa. And using some monad in do-notation, and using the corresponding delimited continuation implementation in normal (non-Haskell) sequential code, will look identical.