At Real Kinetic we've actually been internally using Elm for 2 years. While at Workiva, I OKed some internal projects to use Elm as well. Your observations are absolutely correct.
We have actually found on-boarding engineers to Elm is fast and relatively easy. There is a very predictable learning curve engineers seem to follow. That's one of the things we like about Elm versus some other solutions. The even bigger observation I've made is that all of the engineers we've exposed have become fans, like Alex.
The lack of resources and other companies contributing to the community and libraries is a challenge, and a concern. As an example, we use Elm Native UI for some projects. It has a limited user-base, which has at times been frustrating. We're hoping to see more adoption of Elm to help mitigate this problem.