I could use the opposite : A TODO-app which has no synchronization capabilities so I don't need to worry about leakage of private data. Ideally it would use a very simple plaintext file format for its database so it is future proof, e.g. http://todotxt.org/ The TODO.txt format seems very nice but the Android apps listed on that site are either not fully developed or unmaintained, and one even looks like it's not open…
I just set up a test sync, and the files in my Dropbox Apps/Joplin folder are just Markdown and JSON (there appear to be some dotfiles, but I'll bet they just cache the database graph etc).