I've been using it for about 3 or 4 years. It's very reliable on all platforms, and the developer is responsive to support emails. The app screen shows that I have 80 books (folders), 605 documents (either notes or tasks) and 87,011 words totaling 90MB.
There is no limit to the number of notes you can store that I am aware of, and notes can be almost any file format e.g. plain text, markdown, HTML and you can also import existing files such as PDF, Excel etc which are not editable but will be displayed in the app and sync.
I initially purchased Notebooks App because I wanted a more structured notepad than the standard iOS app (i.e. with folders and sub-folders) and I also wanted to be able to secure notes or folders with a password or thumbprint.
I would agree with the Polish user that you mentioned - it is very much like a Swiss army knife.