Elm lacks extensibility, tooling, and documentation is not that great. The biggest pain point however is the people who run the Elm language. The design decisions they took hurt the language and the users a lot, breaking more and more with every version bump, restricting freedom and creating a walled garden that people are getting tired of.

What you say about JavaScript libraries is not 100% technically correct though. You can still access any native JS library you like, but you got to use ports. You can't hook into native elm functions bound to the global scope, but that's always been a very shady, undocumented and terrible thing to do.

The following reasons are what, I believe, really ruined elm adoption:

1) You can't create so called effect modules (like the http module of the standard library, and so on) if your package is not within the `elm` namespace.

2) As a company, you can't have shared, common elm modules if they are not published in the Elm package public registry. You can't install a package from GitHub without resorting to ugly hacks like ostracized elm package managers written in Ruby.

3) No development timelines, no companies publicly endorsing or using Elm to develop open source libraries besides the one where the language founder is employed.

I've never tried anything purely functional and typed to do frontend programming, so I'd like to hear if Purescript, ReasonML, etc share the same struggles with Elm