I'm actually a Schema user, I love it, it's pretty convenient for addressing the boundary issue, since you don't have types and cannot really enforce that outside input conforms perfectly to the expected schema without run-time validation.

Schema helps (I used to use Mississippi back in the day with all sorts of complex checks, it was ugly), but it's more of an "add-on" in the end. I actually have a reasonably convenient flow for validating outside API input, business logic constraints and performing the necessary operations against the data store all in one big error monad nowadays. The problems is that I still have to go out of my way to add these input/output validation checks to everything when I could just get it for free through the type system.

Look, don't take my word for it: I strongly encourage you to try the two paradigms yourself and see what you think. Unfortunately it's very difficult to see the downsides without a large project, but you can probably extrapolate.