No matter how much beta testing is done, the public at large will always find different tasks that cause problems. Everyone has different hardware and installed software. It's impossible to produce a useful OS that works faultlessly with every combination out there. On top of that you have those that use PCs for purposes they were not intended and blame the problems on the OS.
As soon as the software is released, people are trying to hack it. Any holes have to be plugged. Windows will always be attacked most because it's used by most. Why bother hacking the minor systems? Perhaps for most hackers, Linux is their darling so why attack that?
As far as registry cleaners go, most of what they find is of no interest - empty registry keys, invalid shortcuts, most recently used files. Those are a product of normal PC use and make up 98% of the so-called "errors". CCleaner rarely finds anything, other than after a bad app uninstall, because it is only interested in serious issues. I have yet to download any anti-malware software that has not found something first use and then almost never finds anything else. Funny that. I don't think it has anything to do with how good the software is.