Maximum Type Safety and Error Handling doesn't tell the interesting bit that it uses the type system to track errors. (And I really don't know what Maximum Type Safety is supposed to mean in the first place)

I can imagine, how would you improve it though?

Concurrency, runtime, tracked errors, functional composition, dependency injection, etc there's lots of problems that the ecosystem solves.

It's hard to say which of these features is particularly more important.

I guess to me it looks like yet another monads for JS library. I get its a marketing page but marketing to whom? Maybe there's a better document to share on HN that shows how Effect improves those features?

Algebraic effects are the new hot craze in FP circles. That’s exactly who this is marketing to. People who want monads/pure fp in js.