We're now migrating to React, but have some very large Elm projects still in prod.

The biggest thing for me from practical perspective was to "freeze" some pieces of DOM to be guaranteed to not change/re-created, so that it plays well with some external JS libraries expecting some nodes to not change and stay vanilla.

Another is ability to extend Elm's debugger to filter out big noisy data to keep it usable for our project.

Third is when your data is too big -- it just sometimes fails with "recursion limit" that's hard to debug due to the nature of the langauge.

Otherwise – it's a very beautiful little language that still feels quite modern and easy to work with IMO.