I think what @chenglou was referring to are issues like the "value restriction" [1] and the need to wrap "uncurried" FFI functions with a function that "adapts" it to curried form, thereby adding a layer of indirection.
Reason does not solve these, they're problems inherent to the underlying language and the choices made. I think the point being made was just that currying isn't a clear step forward, but causes some problems as well.