You can create directory in Nextcloud and get credentials that have read/write access via WebDAV only in that directory.

This sounds dumb, but this enables using Nextcloud as a backend for data storage for *any* application in a way that should be reasonably safe.

So you can write an app that can have synchronized storage (configuration, data) without providing a backend service. This is possible with Google Drive and iCloud, but now it is also possible with a popular open storage service.