You might want to take a look at my package gampleman/elm-mapbox, which shows a fairly full featured integration with MapboxGL (which is also a 3D capable mapping library as you probably know).
What I can say from building mapping apps in React and Elm:
- Elm was way more awesome and maintainable in the end. No surprises here.
- The React wrappers we found for Mapbox were always quite awkward, as Mapbox used a bit of a different mental model than React. I have no idea if this applies to Cesium.
- The wrapper library for Elm I ended up writing made #1 above possible, but was a fairly significant time investment (in this case ameliorated by Mapbox providing a decent chunk of their API as a simple JSON schema, so I used code generation heavily).
As others have suggested, the main thing I would worry about is if other developers in the organisation are happy to learn Elm. You can likely hire (especially if you don’t actually require candidates to already know Elm, but rather be proficient in frontend), but you want to avoid a fight with the people already in the org, as that will slow you down.