Both, learning curve was difficult and is impossible to get resources with good/real Elm experience. Also, you end up with a huge Main file in enterprise apps.
Elm use a different paradigm (Functional Programming) and if you are not aware or have experience with it, it will take long time to understand it. The company later moved to React/TS due the issues mentioned above.
Elm is a small and easy language. Certainly much smaller and easier to learn than JavaScript. You should go through the Elm tutorial. It shouldn’t take you long.
And do you think understanding Elm requires advanced skills / more than senior level ?