Besides the fact I totally disagree with robot zombies assessment of CCleaner (I've used it for nearly 2 decades on 100s of computers and never, not once, ever had it ever "bork" a computer beyond what a simple reboot didn't fix - and even that was an extremely rare issue), CCleaner is not "health diagnostics program" so it should never have been mentioned in the first place.
That's funny, because I don't actually doubt or disagree with you. I think used as intended, it's fine. But if you're more of a casual user trying to 'fix up' your PC, caveat emptor.
There is a reason Microsoft advises against using applications that mess with registry entries like that. For all I know, it's different from what I'm thinking, but I have a feeling it's not just about control. It's not like they stop you from modifying the registry however you please. And yet they pretty adamantly steer clear of any apps that claim to fix the registry.
Can't blame em for that. For you or me, it's an easy fix... if not very annoying. But perhaps not something fixable by a reboot... and also something that can be a real nightmare to troubleshoot over the wire, depending on who's on the other end. So in general I tend to steer people who don't already know what it is and what it's for away from it. Basically all it does in my book is simply things that most power users already do as a matter of optimizing machines, so for them it's just a handy gui that saves some time. I imagine that is especially true for a guy like you, who does things like that for a job. It's also pretty aggressive and powerful. It really can get a lot done when it comes to cleanup, optimization, privacy stuff, removing stubborn unwanted apps. Great for slimming down, and really making sure you're not leaving unwanted footprints and orphans about. But for that same reason, people can and do screw things up in ways they don't always know how to fix.
Back in the day I actually used it and found it helpful. Also worth mentioning though... it did do a number on my registry the first time I used it, more than I had the knowledge or patience to fix at the time. A fresh install was easier. So I can say for sure that it is possible. For all I know it has changed drastically since then. Doesn't seem to work differently though. It's still a file and registry cleaner at heart.
Gotta consider that hardware was also a lot more limited in that app's heyday, software was a bit... 'needier' and so there was sometimes a noticeable benefit to cleaning up. Especially for 32-bit, memory limited systems. But these days, I don't see it being necessary, let alone likely to resolve whatever performance issues a person might be having on current hardware. So it's probably not worth the risk of unwittingly deleting or modifying the wrong thing.
Honestly, I think the fairest thing I could say is it had it's time. For some it is still useful, but it's a lot less relevant and still not the best thing for users not versed in registry to be messing with. I'd rather, if they have an interest in making their PCs run better... especially their CUSTOM PCs, that they stay away from tools like that until they learn how to do the things it does in OS so that if something does go wrong, they understand it well enough that it's as trivial for them as it would be for you. At that point I can see the benefit in using it.
That's me. One thing I know we agree on, this is not the place for it.