The better question is, what’s NOT wrong with Nextcloud?
Nextcloud is also a perpetually half made project that breaks at every corner and requires a lot of resources. The sync works until you add like a TB of small files and it never works fine again, unlike Syncthing that can handle whatever you need. Also unlike FileBrowser the WebUI is slow at listing files and related operations. The webmail is yet another pile of JS errors and nonsense idiotic stuff like being unable to show a bullet list.