Couple of points though:
It wasn't clear to me where the notes are actually stored. If I'm relying on your tool to collect my private thoughts, I would require raw access to files. Sure, there is backup & restore features, but I'd still require full content access without going through the client. The info wasn't in any FAQ. It seems to be a SQLite db inside ~/.config/Notesnook/. Ideally the note taking system would have open and documented format so others can build tools to insert and query notes, provide CLI interface, show dated notes on calendar, and so on. I understand this is not necessary the direction you want to take this product.
When importing a .md document, the first line "# heading" was set to be paragraph type with 24pt size instead of Header 1 type, and Header 1 seems to be generally missing from the pull-down menu. Other types of headings were correctly transformed.
As I type in a new heading using the markdown syntax "## subheader" and press Enter, the text is correctly converted to Header 2 type, but Pro version popup is triggered. I would call this a bug. Either don't convert to header, or don't show the nag screen.
The splash screen that advertised the Monograph feature could not be dismissed without pressing "Start publishing" button. Maybe that button should be changed to some less misleading text (it doesn't actually lead to any publishing feature).
I like the business model and I would gladly subscribe at this price. I also love that I can already run it and try everything out without even having an account. The thing is, right now the text formatting buttons are a minefield of PRO-only features. I just dislike when applications include buttons for pro features that open nag screen. If you want to keep the client slightly crippled for free users, you could at least disable (gray out) pro features. It would be much cleaner cut if only syncing service is paid, but I don't know enough about your target users and business model.
In general I would love to be able to restrict formatting UI to just markdown features. I don't want to accidentally change font size or mess with text alignment. All included options for text formatting are just noise to me and I prefer simplicity in note taking.
Just few thoughts from trying it for 20 minutes. I'm still thinking about using it seriously.