In what way should it be unsafe? If you get it from the developer directly, there will be no malware attached to it. As it also only reads your disks, it will not delete any data on the disks you want to scan.
CleanMyMac access the system and makes deletions of probably important files if one does not pay attention, DaisyDisk just reads all the folders and files on the disk you chose to scan. It does not delete anything.
If it would do that, many would know and its reputation would have sunk quite heavily.