On the free tier you have 3 notebooks, on paid it's unlimited and yeah the notebooks are essentially folders. The way I have mine setup is a general notebook for a certain topic and then tags to subdivide; you can also assign colors to notes. Also I should mention, there's a built in publishing / sharing feature that let's you give out links for people to view your notes.

Stacked notebooks are still in the roadmap so it'll take a little while to be able to put notebooks inside notebooks.

True, can't knock you down for sticking with obsidian, it's a really good app and like you said it's already a bit more mature. The way I see it, notesnook is one of those things you keep in your back pocket when you need an alternative, like for me, when obsidian and Joplin started giving me issues and impacting my workflow I needed an alternative.