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Worse than obsidian if you try and use it like obsidian, sure. But better than obsidian at being a Notion replacement for certain tasks.
Same reason there’s dozens of closed-source note-taking and project management apps. Some have different feature sets and workflows that enable different functionality. One might be great for your specific use-case, another might not. That doesn’t mean the latter doesn’t have any merit, or that it won’t be the perfect solution for someone else who has different needs.
Seafile is probably the next biggest/most developed alternative. When I’ve tried setting it up in the past I’ve had serious issues getting it running behind a reverse proxy. Would be fine for local or VPN/Tailscale style usage though. Also, data is stored in a proprietary database which would make configuring external backups a bit harder.
Syncthing is probably the most flexible option, but it’s not a “cloud” storage solution, it’s peer-to-peer file syncing. Doesn’t really allow for easy selective sync or quick deployment and configuration.
There’s also Owncloud but I don’t have experience with it.