I've found usually to poor effect. Both Rust and Haskell did away with .mli files and ended up worse for it. Haskell simplified the boundary between modules and offloaded the abstractions it was used for into its more robust type system, but it ended up lobotomizing the modularity paradigm that ML did so well.

Rust did the exact opposite and spread the interface language across keywords in source files, making it simultaneously more complicated and ultimately less powerful since it ends up lacking a cognate to higher order modules. Now the interfaces are machine generated, but the modularity paradigm ends up lobotomized all the same, and my code is littered with pub impls (pimples, as I like to call it) as though it's Java or C# or some other ugly mudball.

For Haskell, the type system at least copes for the bad module system outside of compile times. For Rust, it's hard for me to say anything positive about its approach to interfaces and modules. I'd rather just have .mlis instead of either.

I don't understand that hate against header files. It is just a separate file with the interface. Of course you also need to change it, when you change the API, which you might find annoying, but maybe you should use that to consider that you are just changing an API?