Some what recently, it stopped clearing my call logs. It says it does find and remove them, but, they're still there in the phone app. There was also some kind of update from AT&T for my Samsung Galaxy S6 today and now it doesn't see any running system or user processes. Phone is still currently on Android 5.1.1, so I don't know what AT&T updated.
Clarifying, my phone's already been on Android 5.1.1 for quite a few months - whatever they updated today is what I don't know about (I got this phone in November).
After the "upgrade" (Android claimed it was upgrading), CCleaner quit seeing any running user or system processes.
Not sure when it stopped being able to clean out my call logs form the phone app... this most likely happened somewhat recently with one of the last few app updates.
Other than those two possible bugs, I'm happy to report the app has always been working as designed and intended!
Wrote all of this as a review (since we can't simply comment) on the Play Store, hopefully it helps the developers.