Elm has an amazing time-traveling debugging system: http://debug.elm-lang.org/ This is a cut above pretty much every other front-end JS framework, and languages that transpile to JS.
> the lack of third party libraries
It has JavaScript interop that's based around message-passing. The JS library is treated sort of like a service, and thus the Elm code is kept isolated from it: https://guide.elm-lang.org/interop/javascript.html
> the increased difficulty to find good training courses for new junior developers you hire
The official docs are pretty good. A good programmer shouldn't have a trouble picking it up. The architectural benefits of using Elm (that lead to a high-quality, almost zero bugs website) is IMO worth it.
Elm also guarentees to NOT produce ANY runtime errors, that's quite a guarantee.
I mean is the time you and your coworkers will need to learn it, the increased difficulty to hire skilled elm developers, the increased difficulty to find good training courses for new junior developers you hire, the increased difficulty to debug the code compared to js, the lack of third party libraries ... really worth it?