The links are composed of over 60 randomly generated characters. The likelihood of someone guessing a shared link is extremely low. As long as you only share the link with people you trust, your shares are secure.
In addition, as a myCloud Standard and Pro customer, you have the option of protecting shared content from the "Files" area with a password or limited sharing period. For myCloud Light customers the link expires automatically after 30 days and it is not possible to set up additional password protection.