I heard sometime ago that given that Elm is build around FRP, is not very good for making widgets or reusable and nested components. Why would this be the case?
Having used Elm a little, the aim certainly is completely opposite to that. Moreover, Elm is mostly pure even in its handling of the reactive paradigm. That alone should make it more composable than most other languages. Should, because as far as I know it hasn't been proven just yet.