A confirmed, yet still not resolved bug caused me and about 200 other people lose data (metadata) for tons of files. Well, at least 200 reacted to the GitHub bugreport I filled. I think you can easily find it because it's the most upvoted yet unresolved issue.
Besides this, it'd often give random errors and just not function properly. My favorites are the unexplained file locks: My brother in Christ, what do you mean error while deleting a file. It's 2023 holy shit, just delete the damn file. It's ridiculously unreliable and fragile. They have tons, thousands of bugreports open - yet they focus on pushing new, unwanted social features to become the new facebook and zoom. They definitely should focus on fixing the foundation first.
https://github.com/nextcloud/desktop/issues/4378