With DAVx⁵ (https://f-droid.org/packages/at.bitfire.davdroid/) I find contact and calendar sync with Nextcloud to be quite reliable.

That said, for file syncs I do prefer Seafile over Nextcloud. I don't think I'm the only one that prefers Nextcloud just for contact/calendar sync because Mailcow uses the Nextcloud platform just to accomplish that, with file sync disabled.

On desktop the sync integration is fine. It's only on mobile that the experience is lacking.

Try feeding it a redis instance. That caused a "several orders of magnitude" improvement in interface responsiveness and my weird syncing problems stopped immediately.

I also disabled every module and plugin I didn't consider essential.

Same here, and I have many problems with the web interface too. And sometimes when I add an app to my Nextcloud user, the whole instance just stops working and I wasn't able to find a way to recover.

Nextcloud totally breaks down if you try to use it for photo syncing. It can’t handle thousands of files in one directory.

Hows Owncloud doing? They were rewriting to Go, to improve performance in such cases. I kind of lost track though.

The impression I got was that the problem was not related to php being slow but just a general scalability problem. If go runs it 2x as fast, it will still fail with 2x as many photos which is very common. Maybe as part of the rewrite they will address this but I think photos is a fundamentally different problem to hierarchical file storage that they require being split up like how google photos is not part of google drive despite sharing the same storage usage quota.