Well, the only paragraph of such System Utilities that is might be lethal for an OS - is a registry cleaning module. None of other. That should be used carefully at a start (when you test-driving the program), making checks and notes.
What about driver updaters? These can also be "lethal" for a Windows system.
Ashampoo WinOptimizer, Glary, YL WinUtilities contain "Disk Cleaner" which cleans out most of junk redundant files off from your System, like rests from uninstalled software, etc. Also, there's an "Internet Cleaner" to clean out all sorts of a browsing data left by a browser, like Firefox, Chrome, IE, Opera and Safari - its somewhat replacement / or if you don't want to do it through a browser's main window, re-opening it again.
CCleaner, TFC and Windows Disk-Clean Up.
These are main..... all of these include a disk defragmentor, which is and should be used on a regular basis. Ashampoo offer a really spin one, fast.
Windows have in-built disk defragging, or you could use a program dedicated to that, no need to install a whole bloated suite for that.
These, and only these packages include a "File Splitter and Joiner", a "File Encryptor" - some are even military-grade. It's good to have a method and technology to pack your best files with a military-grade encryption and a password as a bonus, isn't it?
TrueCrypt, DiskCryptor, VeraCrypt, etc. are all better than the "disk encryption" features of these programs. Hell, even Windows BitLocker is better.
iolo System Mechanic is beloved and trusted by millions, as they say, as a number 1 from others, offering all those functions too since early 2000s (a very popular and famous software). Their code and methods for an OS and files are truely amazing and remarkable, elite and a top-class. It can be best described as a standalone package for all of your computer needs, for any user (intermediate, or so).
It's super easy to say that a product is used by millions and to make up numbers. Unless this was reported by a third-party that have no direct link with IOLO and that can provide actual statistics, I won't believe that.
TuneUp and Auslogics are also a great products that are offering all that as well. Australian Auslogics are somewhat greater and more elite, also costly - around $50 or less; their latest version looks a bit like a Mac software.
Guess what, AVG PC TuneUp is always the first program that cause system issues under Windows and when I help someone that have it installed, uninstalling it boost the system performance and solve the random issues he's having in 90% of the case. Not even being overealistic here. Oh, and I can also do everything, every-thing, these programs do for absolutely free, in a safe and efficient way.
rhino, no actual technical support or IT professionals are ever, ever going to recommend these programs or will say that they are useful. And this is a fact coming from an actual IT tech. support (me).