In Sandstorm.io's case, I use an app called Davros to store files (which is even compatible with some ownCloud sync clients, actually), and Cloudron.io has a few other options you can use to work with your files, including, actually just hosting Nextcloud or ownCloud on it.

Where these sorts of platforms really excel is in the realm of other features, which Nextcloud tacks onto the existing platform, but a larger app platform can allow you to use a variety of alternatives which are more tailor-made to other specific types of web apps.