Prior to the most recent Windows 10 Insider Builds (14942(?) - 14965), CCLeaner removed Windows.old completely. No leftovers, just check the appropriate box under Advanced and CCleaner performed perfectly.
A new Windows 10 Insider Build has just been released: Build 14971. In the Release Notes, Microsoft states "We fixed an issue that could result in sometimes not being able to completely remove files in the Windows.old folder using Disk Cleanup". So it looks like this was a bug in a few of the pre-release builds and not a CCleaner bug.
I will run CCleaner to remove Windows.old after Build 14971 has installed to check that this bug has been fixed. I'll run Windows Disk Cleanup on one of my computers and CCleaner on another.