I personnaly don't like the single enum model for messages. I prefer the generic Handler trait model.

Also another square that I have not circled with async actor other than actix is that all of them use mutable self and they dont have a great way to have long running tasks. Sometimes you would want to call a remote server without blocking the actor, in actix this is easy to do and you unamed child actors to do that. All those newer frameworks don't have the primitives.