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.