I have been using Registry Mechanic v7 (The last one released before Vista) for years and have never had a single problem. As I said before though, I wouldn't recommend any version newer than 7.
Using a registry cleaner can be dangerous. Using one that dates back to 2008 really makes no sense to me. Sorry
I use that one because it's the last version that wasn't changed to be compatible with Vista. They also added a bunch of different features to it in v8 to make it more like CCleaner I guess and they made the registry cleaner a lot more aggressive. I really only use it occasionally because of it's registry compaction feature. Now that I've switched from Auslogics defrag to the free version of Puran , which defragments the registry and even the MFT and pagefile when you do a boot defrag, I may not need Registry mechanic any more. I do use the registry cleaner in CCleaner on a regular basis after uninstalling anything or after removing Windows Update uninstallers after a period of time with no problems. I also periodically will run Auslogics registry Cleaner. I have never had a problem or had to use a backup from any of those programs. I actually have never even backed up a registry clean in CCleaner. That's how much I trust it.