CleanMyMac X is a controversial app on these forums due to its aggressive marketing. That said, it is published in Apple's App Store.
I'm still trying out the app myself, and for now I can say that their Applications Uninstaller seems safe to use. It clearly discloses the files it wishes to delete, and lets you choose if you want to preserve any files it selected for deletion. Also, it may let you reset certain third-party apps by deleting all of their settings and internal data, while still preserving the app itself.
As for the rest of CleanMyMac X, here's what I would advise for now:
- Stay away from the Smart Scan feature, unless you're very familiar with the app, how it works, and what it does. It is safer to treat apps as tools or utilities, where you are in full control.
- If you're using a Mac with Apple silicon, don't use the Shredder. The Secure Enclave already applies per-file encryption, and every file is already securely erased simply by deleting it and emptying the Trash (which destroys that file's encryption keys). Also, the Shredder can perform unnecessary writes, which isn't healthy for an SSD (AFAIK).
- Space Lens seems like a great feature that can help identify large areas of used space (especially if Apple menu -> About This Mac -> Storage is showing a lot of "Other" or "System Data"). However, don't use it to delete any files. Instead, identify which app the files belong to, then use that app to safely delete those files. For more info, see this user tip: What is “Other” storage on a Mac, and how… - Apple Community
- Large and Old Files also seems great, but exercise caution in what you choose to delete.
- Mail Attachments, Trash Bins, Uninstaller, Updater, and Extensions seem safe to use. Probably Malware Removal as well, but a lot of us here recommend the free version of Malwarebytes instead.
- Stay away from the rest of the features (especially System Junk, which deletes important caches) unless you know exactly what you're doing.