I've only used Elm for a hobby project. Given how much I enjoyed the experience of web app programming for once, it's nice to read how well it scales to a large application.
It definitely scales. There is an 'adjustment period' where I went from a nested to a flat, decoupled architecture. But as most things are one to two levels deep (state), complexity also grows fairly linearly. This is really great from a developer's pov when trying to grapple with how to add new features.
This is also my experience in both elm and (type|Java)script/react/redux apps as well: flat decoupled architectures seem to scale better in terms of development/maintenance in addition to reducing (bad) churn. If styling is properly handled, feature progression is also linear-ish.