Dec 21 2020
Just wanted to add that as at Dec 2020 myself and my wife are unable to sync a single list on ToDo on our iPhones.
Having been forced off Wonderlist, it is unacceptable that we do not get at least as good if not better functionality. It was extremely easy to share lists in Wunderlist.
We have both updated the app, and tried logging out and back in. We have also both used the 'Sync' button in settings.