Being able to change the ordering of effects on the fly is a benefit of algebraic-effect systems. As you mentioned `State<S, Result<E, T>>` and `Result<E, State<S, T>>`have very different effects. Algebraic-effects let you switch between the two behaviors when you run the effects, whereas with monad transformers you have to refactor all your code to use `State<S, Result<E, T>>` instead of `Result<E, State<S, T>>` or vice-versa
You can recover the ability to reorder effects by using MTL-style type classes, so you could write that as M<T> where M: MonadState<S> + MonadError<E>, in rust-ish syntax. But that makes the number of trait/typeclass implementation for each transformer explode (given a trait and a type for each transformer, it's O(N^2)), whereas algebraic effect systems don't really have that issue. I also have a hunch that algebraic effects(or, well, delimited continuations in general) are probably easier to optimize than monad transformers, too.
