Personally I think that the unused extensions is one of the safest issues to fix, as long as you look at what you're doing. I have never seen a 'dangerous' extension offered for removal by CC. I think that removal of the 'Fix All Issues' button would be a safer bet, but would result in many complaints here.
I agree that unused extensions is very safe if you know what you are doing. Very, very safe. Because you know what you are doing.
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But there are 3 problems with this.
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A - On systems with a ton of extensions suggested for removal, it will take nearly forever to check & see what should or shouldn't be removed.
B - All it takes for .MP3 for example, to be suggested as safe to remove, is for you to install something like KM Player. Let it take default for MP3. Delete or uninstall KMP & your .MP3 is definitely needed, no doubt. But CCleaner thinks this is now safe to remove since nothing points to it any more. Certain malware samples can also cause CCleaner to erroneously believe that .EXE is safe to remove.
C - Not every user is as tech savy as we are. Therefore, not everyone "knows what is safe to remove". Which makes it very dangerous for some people.
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I would say that if the check for unused extensions is removed, that would make the Fix All Issues a lot safer to use.
Because I have seen a lot of problems from that 1 key check alone.
Oh no, it doesn't happen over night a lot of times, & sometimes the extensions to be removed are chain linked. That is to say, CCleaner removes one extension that another application had a dependency on. Then, because that dependency chain is broken, CCleaner also suggests yet another to remove. In this way (& usually over many consecutive runs), CCleaner can absolutely trash a machine.
Sometimes just by removing an extension, CCleaner will then think that other entries related to that, or that used that extension may be safe to remove their settings. A lot of people don't catch these, so their system degrades over time & they don't know why. So all that head scratching, & they have to reinstall Windows. Wonderful.
Now, I realize you never had problems, but you have to realize that with you only using MS Office, Nero, CCleaner, Firefox, etc, you may not have problems. Some people use/test many times more, or use a lot of "non standard" software in addition to the times someone tried to run a System Restore only to have an incomplete restore because it was corrupted, or to see those [] entries in the registry. Yes, it can happen!
Hence, the problems!
Hope this defines the issues so that you can more clearly see what I meant.
Don