Dropbox, on the other hand, was designed around files, both from a UI point of view and an API point of view. This means their file syncing is very, very good. If a file gets put into a Dropbox somewhere, it ends up everywhere quickly, basically with absolute reliability… If you’re building an app that needs to sync, that kind of reliability is exactly what you’re looking for… Apps can build better UI on those files whether they’re stored locally, stored in Dropbox, or stored in iCloud. But Dropbox has proven it’s reliability, and iCloud hasn’t.
So while there is an argument to be made that Dropbox’s UI is a relic, its value as a syncing engine is still huge, precisely because it’s built around the paradigm of files, a paradigm we have decades of experience working with.
Steve Streza, Files as UI vs API