Two questions come up time and again: "when will the book be updated to Cats 2?", and "when will printed versions be available?" We're happy to say the answer to both is: soon! We have started working on updating the book. In fact the current version of the book, which is linked from the web site, is built against Cats 2 (although it does not discuss new concepts like the Parallel type class). This issue tracks our progress on the update.
One of the most famous and longstanding limitations of the Cats Effect IO type (and the Cats generic typeclasses) is the fact that the only available error channel is Throwable. This stands in contrast to bifunctor or polyfunctor techniques, which add a typed error channel within the monad itself. You can see this easily in type signatures: IO[String] indicates an IO which returns a String or may produce a Throwable error ( Future[String] is directly analogous).