"Does that mean they're all scams?"
Yes! Perhaps my use of Scams is a bit harsh. The problem is, how do these utilities know which registry entries are needed and which are not? The registry has hundreds of thousands of keys and values and a 'scan' does not tell us what the scan is. A scan of the registry? Or a scan of the applications that plant keys and values in to the registry?
If the utilities simply scan the registry, then what are they scanning for? Nil entries? 0 (zero) values? If so, damage can be done because many of these nil entries and 0 values might be required. How does the utility know?
If it scans applications, then the scan is going to take days and weeks and even then may fail. Registry entries are nearly always inserted during Setup, (eg the setup or install file that we 'run' to install the application). Those setup files are often discarded, or are on the CD that the software came on. So the scan will not catch everything. But even there, even if the scan searches through installed .exe files for registry action, then that needs de-compiling of the .exe files, and that is serious hard-core programming and scanning. It is like looking through an applications code for errors and this is something I had to do years ago when I did programming myself in simple BASIC language. Such files were typically only 2 or 3 Kb in size and troubleshooting would often take me weeks, even though I was the one who created the code anyway.
So I don't even begin to believe that such scanners are scanning applications for their relevant registry entries, then to dismiss those as proper so they make a note of the registry entries not assigned to any software.
Whatever method they use, (I discount some sort of internal database as that would probably take a computer processor with the capacity of Marvin to accommodate, (who had a brain the size of a planet) - Hitchhiker's Guide to the Galaxy), they are going to get it wrong, and here's the rub; it only takes one change in the registry, one wrong deletion or value change to kill the OS stone dead.
So, may be not scams, as they may be well meaning. But utterly useless and all too often very dangerous.
The registry is itself just a database and as with all databases they are static. They don't do anything except hold data. So any unused or unwanted data is simply ignored. That might increase the size of the registry over time, but compared to other files on the hard drive the registry file is tiny and today's processors can whip through them in less than a blink of they eye.
Without being rude, I would say that I was a little surprised by your statement that, "In following up on some people's opinion that registry cleaners as a group are scams, I decided to try Glary Utilities and Iolo's System Mechanic".
I have to wonder why anyone who read all the problems and opinions would then download, install and try 2 other recognised 'never recommended' registry scanner/optimisers. So now you have 3, you are using all 3 one after the other, and they all report different results.
The regulars in these forums never recommend any utilities like these, so my own advice would be to uninstall all 3 and trust that no damage has been done.
You may get other, differing, advice of course.
Just a "heads up" for anyone considering Regzooka: I bought a license for Regzooka that was to cover 3 machines. (more about the performance of the product at the bottom) At the time of purchase, I also decided to try their "free" month of something they called Cyber backup. (never used it)
I've just noticed they have been charging me a monthly fee for this Cyberbackup utility for who knows how long. I called them, and their response was something along the lines of: "if you read the text at the bottom, it says that after your initial month, you will be charged a monthly subscription rate."
I think this is sneaky, and although they did consent to refund 2 months, their attitude was the "you need to pay closer attention to the fine print" type. Well, I've learned my lesson, and I hope to spare others from falling into the same barrel as me.
As far as the performance of Regzooka itself, I'm not sure what to think. It SAYS it's searching, and has found X amount of registry errors, and "Presto! at the click of a keystroke, they're all gone, and life is good!
In following up on some people's opinion that registry cleaners as a group are scams, I decided to try Glary Utilities and Iolo's System Mechanic, both of which do registry cleaning. At various times over the past several weeks, I periodically run Regzooka and "clean up" all my registry problems. Then I run Glary utilities, and that program finds X more registry errors, then Iolo then finds even more!
Now I'm no expert, but if Regzooka says that it has done a complete registry cleaning, and then 2 other programs come right behind and find 20-30-50 MORE errors, then something's fishy. And to be fair, I did these experiments in different orders, and the results were the same: After the first "cleaning", the other programs can still find and fix a bunch of "errors."
Does that mean they're all scams? <shrug>?