"known to be incompatible"

Known to whom? People who regularly participate in the Julia forum/chat? Julia's composability relies on people agreeing on unwritten rules and standards.

In other languages, such incompatibilities are caught by the compiler. Even in other dynamic languages like Python or Javascript, it is now considered best practice by many to annotate types whenever you can. Like Julia, Haskell is also composable. Unlike Julia, it does not need to sacrifice correctness.