Hello there!
I have three PC's, all with Windows 10 and CCleaner.
I used CCleaner on all of them, and it worked just fine. However, after an issue with one PC I had to reset 10. This was the Microsoft Reset process that effectively re-installed 10. The process left the old version of Windows 10 on the PC as well as the newly "reset" version.
I used CCleaner with great success but then decided to use the option "delete old OS" in the CCleaner options to get rid of the damages 10.
This was a terrible mistake, lol. It apparently completed, but after the PC was restarted it fatal errored. The damage was so severe that Windows 10 could not repair it. The PC would not boot and could not be repaired. It even said that it could not perform a reset. In the end I had to re-install 7 and then upgrade again to 10.
At this stage I would STRONGLY recommend that people don't use the option to delete old versions of windows!