I don't get the "note data is stored in a proprietary format" hate. There's a dozen different open source tools you can find which will one-shot export your Apple Notes to markdown, when you want to leave the ecosystem. Its not like Notion where the exports are messy, non-bulk, and destructive; they're proprietary, but parseable and sitting in a file on your MacOS filesystem.
I've never had iCloud Sync display any weird behavior in Apple Notes, and I've got thousands in there. I have, absolutely, seen Obsidian Sync delete notes I did not delete, and fail to upload notes; both of these were generally remediable via their recycle bin, but still very concerning.
All of these complaints are of the nature of "I don't actually have anything valuable to write down so I'd rather worry about the nature of the tool than what I'm writing". And, to be clear, I think this is why Notion is so popular, just for different reasons than your's; it looks great, and makes you feel productive because you've got amazing cross-referenced tables and hyperbacklinks and h1h2h3s and then wait where's the content?
Counterpoint, I have seen sync issues on very popular large note, cloud-based solutions that use proprietary formats - Evernote comes to mind.
When you have tens of thousands of notes, it's hard to even know if at some point, the note has been suddenly changed, reverted, or modified in a way that you didn't even realize occurred.
With an open format, specifically a text based one like Markdown, I can sync all my notes to a git repository in a diffable manner that I can quickly review.
I've never had iCloud Sync display any weird behavior in Apple Notes, and I've got thousands in there. I have, absolutely, seen Obsidian Sync delete notes I did not delete, and fail to upload notes; both of these were generally remediable via their recycle bin, but still very concerning.
All of these complaints are of the nature of "I don't actually have anything valuable to write down so I'd rather worry about the nature of the tool than what I'm writing". And, to be clear, I think this is why Notion is so popular, just for different reasons than your's; it looks great, and makes you feel productive because you've got amazing cross-referenced tables and hyperbacklinks and h1h2h3s and then wait where's the content?