Let me tell you what I'm worried about with effects systems: One of the smartest people I know of has a library called Eff https://github.com/hasura/eff that efficiently implements effects through some modifications to GHC. The thing is, she's pretty much stopped working on because she found some really nasty semantic edge-cases that she couldn't resolve to her satisfaction. (I don't understand the problems well enough to describe them, and I think they're specific to lazy languages, but it has left me cautious about the effect model in general.)