Offline-first isn't enough.

I've done user support with users in the mountains who don't have a reliable internet connection.

Being able to say 'don't worry, the app works offline, you can (optionally) sync when you're next in the city' is extremely rewarding, and vital for software to work for these people.

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Offline-only is not enough either. Ideally users should be able to sync between devices when offline, and have the option to sync to the cloud when online