I use syncthing for notes across iOS and macOS devices and I wouldn't say it's great. It works most of the time until it doesn't, until one client all of a sudden stops syncing and then you gotta figure out where it's hanging. Couple of days ago it was the macOS syncthing app. Just wouldn't start, wouldn't show an error. Usually it's the iOS helper app "Möbius Sync" not actually syncing in the background. It's annoying but still better than using a cloud provider.

I suspect the experience is better on Android.