All of my devices are running the latest OS.
The syncing is inconsistent at best, but here are some examples of what happens.
1. Add a new item to an EXISTING To-Do on my Mac, close the application.
2. At a later time, I try to view the list on an iOS device, but I'll notice the item hasn't been added. Sometimes, I'll close the app, reopen it, and a partially updated list will appear. For example, if I added two items on my Mac, only one of the items will appear on the iOS device.
3. When I try to add or remove items from an EXISTING list in iOS then at a later time try to view the list on my Mac, I'll notice that the Mac list hasn't updated. This can be frustrating because the older Mac list might replace the list created on the iPhone, so when I return to the iPhone, the list updates and uses the older list. For example, on the iOS list I'll mark two out of three items as complete. Later, I'll open the list on my Mac, but all three items still appear as needing to be done. If I close that list without removing the completed items and then return to the iOS device to view the list at a later date, the list from the Mac with the three original items appears (of course it seems to sync fine when errors are involved!). To-Do is supposed to help me keep track of what I've done and need to do, but when I have multiple lists with many items, I can't keep track of which list (Mac or iOS version) is the most up-to-date when these kinds of syncing errors occur.
Please let me know if you need additional clarification.