It isn’t open source, but it writes standard markdown files to disk, so I can switch programs whenever I like and I am not locked into the Obsidian ecosystem with my notes. That was the main reason why I decided against using Joplin, especially after my experience with converting recipes from Nextcloud Cookbook to markdown …
In general I am always trying to find a simple file-based solution for whatever I need to do. I want to be able to sync it with Syncthing instead of something fancier that requires a centralised web server or even relies on a cloud service.