For me the issue is “importing and exporting”. I just don’t want to have a note-taking software anymore where I can’t just read or edit plain text files with any text editor I happen to have.
I know I can export my notes from Joplin into markdown, but when I last tried it, I wasn’t satisfied with the result. I don’t remember it anymore, but exporting either didn’t preserve the file hierarchy, caused issues with linked images or I had to do something else with the markdown files.
I also didn’t like that Joplin had to sync with a local folder instead of just using the files directly. Overall it was just too complex for what I need (i. e. a glorified text editor).
But I don’t want to discourage anyone from using Joplin. Different people simply have different needs and Joplin is free to use so one should try it out and see for themselves. ;)