alt Hacker NewsI never had data outright vanish, but similar to the comment you replied to, it was just unreliable. I found Syncthing much more useful over the long haul. The last 3 times I've had to do anything with it were simply to manage having new machines replace old ones.
Syncthing sadly doesn't let you not download some folders or files, but I just moved those to other storage. It beats the Nextcloud headache.
I might be misunderstanding what you mean, but maybe the .stignore[1] file is what you're looking for? Apologies if it isn't :-)
[1] https://docs.syncthing.net/users/ignoring.html