I don’t think it’s a great option privacy-wise. Privacy policy: Notion
and TOS;DR page: Terms of Service; Didn't Read - Phoenix
However, I needed something free and accessible anywhere that I could use for “personal” work notes that wasn’t tied to an app or local files, so I use it for my job. I chose it over more obscure options at the time just in case anyone asked about it, and because some of the competitors I list below were much more new at the time.
Private apps that are very Notion-like would be Anytype.io or Appflowy. Obsidian/Logseq are pretty different, though Logseq has ‘blocks’ like Notion.
*forgot about AFFiNE (another notion-like app)