I don't think there's anything wrong with the software itself, but was slightly surprised by the files/folders it offers to get rid of.
I rarely use cleaner or optimization software. I don't install software often, & the apps I use I'd use regardless their footprint, so no use worrying about it. ¯\_(ツ)_/¯ I run my web browsers in private mode, [allegedly] deleting their cache when closed, and have a list of folders I regularly make sure are empty, like the temp folders & Windows\ SoftwareDistribution\ Download. I archive quite a few large setup files from ProgramData & Windows\ Installer to another storage drive, along with content folders installed by whatever apps when possible. While I have System Restore on, I delete or clear any Restore Points regularly. And I regularly use Disk Cleanup & dism /online /Cleanup-Image /StartComponentCleanup.
But I was curious what WDC would/could do, so I monitored what files/folders it deleted when I selected every option it offered in my Win7 32-bit VM. Some of the results shouldn't be a problem, like sample images & files related to additional languages. However I didn't find any references, or anything about potential problems deleting the Default & All Users folders -- these were Not deleted running WDC in my Win10 64-bit VM. The easy, potential problems I can think of are: installation routine for some apps might add some of them back, or I imagine fail -- some Start Menu shortcuts would go missing -- some apps would fail, needing data they store there.
Deleting the C:\ Documents and Settings shortcut doesn't so much, but deleting System Volume Information is AFAIK unheard of, and probably useless as Windows will just grow a new one. It **may** just be their way of deleting all restore points? If you use WizTree there's a good chance you'll find lots of stuff in ProgramData caches, like often copies of setup files, and the disk space they use can be fairly large. In the Win7 VM those were left alone, though the scan in Win10 did include some, but missed what I thought were obvious candidates, like C/C++ runtime installers. The Windows\ Installer folder is emptied. That shouldn't cause any immediate problems, but files stored there are used if/when you uninstall software -- if you have any apps with a large footprint I'd definitely think twice about that one. Clearing out the Windows\ WinSxS\ Backup folder is, like deleting System Volume Information, AFAIK unheard of. I feel it probably, kind of makes sense, since WinSxS grows quite large, but Microsoft knows that its size is a problem, & so it does a lot to keep it as small as possible. I don't know exactly why that backup folder exists -- I didn't find any documentation -- but with everything else Microsoft has done to keep WinSxS as small as possible, there's a decent chance there's a good reason for it.
That said, there is one folder I'm surprised that WDC left alone: C:\ Windows\ servicing\ LCU\. In Win10/11 that folder stores files from previous cumulative updates -- Disk Cleanup &/or DISM's Cleanup-Image will delete all but the latest one. Still, that one folder, Package_for_RollupFix..., often holds 1GB+. AFAIK there's no official documentation on deleting it, but there are also no reports of anything bad happening to those who have deleted it.
