I have been using NC for a few years now and I am mostly happy with it. Contacts, calendars, sharing, photo auto uploads all work with my Android phone.
My problem is the software is pretty slow and from a security perspective I'd be hesitant to expose it to the greater internet. That's just my gut feeling.
I feel the same. Basically, I wouldn't like to publish the address of my personal Nextcloud server to anyone (unless I am sharing a file privately with a friend). Fortunately, the social features are not built into the Nextcloud core, rather they are in a separate app. So we can just forget about it if we don't like it.
If you're not sure about the security you can firewall it of course, but keep in mind we pay 5K to anyone who can find a remote execute vulnerability in Nextcloud (and smaller amounts for smaller bugs).
My problem is the software is pretty slow and from a security perspective I'd be hesitant to expose it to the greater internet. That's just my gut feeling.