Salesforce has the following limitations for masking tasks:
•You cannot apply filters on the TextArea and TextEncrypted field types.
•Salesforce has limit of 20,000 characters on the size of a query. You can configure source or target profiles to hide the fields to reduce the number of fields in a object.
•You cannot perform an upsert operation if the Salesforce user account does not have creatable or updatable permissions on the external ID.
•You cannot use the User, UserLicense, Profile, Community, and Group objects in a masking task.