For me monads are similar to inheritance. There are areas where one topic/functionality is dominant and it can really help to define a base class in a library or define a monad like for async. The moment you start to mix/compose things, things get ugly pretty fast.

You can't just put assignment in a functional language though - you lose the ability to fearlessly refactor that's the whole point. You either need something like a stratified language (which I've never seen actually implemented, much less production-ready, as much as I like the design of Noether), or you use, well, monads.