It's for contacts, calendar, tasks, and notes. It's more of a CalDAV/CardDAV replacement with end-to-end encryption than a Nextcloud replacement, which AFAIK doesn't have E2EE for syncing these bits.
Also, as it doesn't directly use CalDAV/CardDAV it depends on a client app. At least on Android, the app syncs to the system calendar/contacts so it works just as seamlessly as Nextcloud's recommendation of DAVx5 or any other contacts/calendar/tasks service in that you can access the data from any standard calendar or contacts application.
So it is basically the same... except that you can't share pictures, videos, or other file types? I wonder why you need a DAVx5-like app with EteSync if that is all it does?
It's not basically the same. Encryption makes the whole difference. With NextCloud all of your data is accessible by your provider, its employees, hackers, and everyone who gains access to the server. This is not theoretical, hacks happen all the time.
Not everyone is a techie and can self host. You'd also need to self-host at home (even more of a pain). And anyhow, even if you are hosting at home, an internet connected server is much more of a target than a usually-behind-NAT phone.
It is not externally accessible... only the home VPN is (I'm also using DDNS just in case my semi-static IPv4 changes). But of course nothing is 100% secure.
Sure, if you have a VPN that's probably fine. Though this is another piece of tech you need to setup, and thus another barrier for non techies. That's why encryption (secure by default) is better than having users know how to securely setup things, and even needing to know that it's a concern.
