I have used CCleaner for well over a decade now on Win7, Win8, and Win10 and love it.
I'm not using the registry cleaning feature as much, and not at all yet on Win10, but have yet to have any system malfunction subsequent to using the Registry Cleaner option. There seems to be a consensus in "the geek community" that registry cleaning is unnecessary, and perhaps it may be.
It is one of the easiest ways I've ever found for doing "housekeeping" and getting rid of all sorts of temporary/stray/orphan files left behind cluttering up the drive. It's my favorite way to clean browser cache and gets that job done more quickly than the respective browsers' built-in functions. It's also very handy for disabling and/or deleting startup programs with much greater ease than digging under the hood with Windows.
I've used it on my partner's Win8.1 laptop for both disc space cleanup and registry cleanup for a couple of years now. There have been no problems as a result.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Presenting the willfully ignorant with facts is the very definition of casting pearls before swine.
~ Brian Vogel