Interesting. I designed, fabricated, built, installed and used my first computer in 1975. My first PC in 1985 or thereabouts. I've worked in all positions, helpdesk, manager, database creation and support, network and internetwork design, etc., etc. I've never felt the need to prove that a registry cleaner is worthwhile; it either increases performance or it does not. Yet I have to wonder . . . a registry that is, say 1.2 Gb, with numerous unneeded and dead items, must take a little longer to load, parse, and run than a registry that's only 760Kb. Can anyone point to research that proves or disproves this conjecture?
That was exactly what I was trying to say earlier. The registry is a database and if it gets bigger, it will obviously slow down. No different than my gmail.com on the web or even using Thunderbird. The first page is fast which is 100 messages but if you go to like 10+ pages, it will slow down. If I remember correctly and not sure if it's true or not, the registry is supposed to grow even if you clicked the mouse once so it all adds up over the long term. I mean even with regedit, a new system when you search in it will obviously be way faster than a system with a registry that is like 5 years old. Obviously uninstallers like Revo Uninstallers work as a good case is Dell Trusted Installer v6.3 to v6.4. v6.4 if you upgrade will be stuck on the splash screen so one will have to reinstall v6.3 and then Revo Uninstall v6.3 completely before installing v6.4 where it will work correctly as v6.3 calls it Dell Trusted Agent while v6.4 has a different name. Others had the same problem.
As for registry cleaners, while I do have CCleaner installed since the 2000s, I had never used it.
The Norton one that originally came with Norton Utilities seems to be the safest because it just does the cleaning from the start to finish and backs up on it's own so you can undo it. Norton does not call it a registry cleaner but more registry repair which seems to check keys to see if they are needed anymore and also compresses the registry.
jv16 PowerTools which was what people used in the 2000's basically scans and let's you select individually what to keep or remove and it also backs up the things it removes but is more risky because it seems to have way more things than Norton would show.
As for not being able to boot up, couldn't one just boot up in Safe Mode in Windows, run the same registry cleaner and then just undo the changes using the backups provided or the other method is to first do a system restore point, use the registry cleaner and clean whatever and if it fails, then just restore the system restore point.
jv16 also has a compress registry function. CCleaner does not have a registry compressor function.
I mean there was one time where I couldn't get a Intel Rapid Storage Driver never version to install so I deleted the current used driver in Device Manager, Storage including deleting the actual driver files and yes, it BSOD/GSOD but Windows Automatic Repair took over and at the end, system was back to normal again. Not sure what it did to restore the driver I deleted. I forgot the major no no from Windows XP where you can't just clone the HDD with the Intel Rapid Storage RAID driver and expect it to boot on a different motherboard without first switching to Standard IDE for the Storage Controller first in Device Manager before cloning or else it will result in a BSOD.