DOES NOT WORK!

The lists would not sync between my Mac and iPhone, so I contacted support. The creator’s response speaks for itself: Hi, thank you for contacting us.
Actually we use old, currently deprecated, iCal API to access Reminders, so it is not guaranteed to work for all calendars.
I.e. it works but we do not have any way to fix or improve if something goes wrong.
Also currently we are working on To-do Lists 2.0, which will use new EventKit API, and will work much better on syncing then To-do Lists 1.x

Constable711 about To-do Lists, v1.7.7