I tried the app on Android and Ubuntu, looks both good 
But as already mentioned by @gverdun the main downside for me is the cryptic file names it creates. This makes it impossible to read the notes without the Joplin app because you can’t find the right file without opening them one by one.
What I also don’t like is that the created notebooks are just another reference file instead of a sub-folder, but I guess the reason is for compatibility with different storage options where folder creation might not be supported.
So for now I will continue to use different apps on web (NC Markdown app), desktop (Typora) and mobile (still trying…)