my understanding is that syncthing doesn't work properly on iOS, because Apple doesn't allow background processes. So you have to manually sync all the time. I'd imagine you'd get into some merge conflicts if you forget to manually sync and start editing notes.

That's why ios apps generally either use their own sync, or use iCloud. As someone that has a mix of devices (iOS, Linux desktop, macbook for work), this leave me with very few cross-platform options.