I stopped using Nextcloud when the iOS app lost data.
For some reason the app disconnected from my account in the background from time to time (annoying but didn't think it was critical). Once I pasted data on Nextcloud through the Files app integration, it didn't sync because it was disconnected and didn't say anything, and it lost the data.
I 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.
Oof, sounds painful. It's hard to use anything when you can't trust its fundamentals.