The consultant should recommend using either an email address or a CRM ID as the subscriber key for the cross-channel campaigns. Email addresses are a more reliable option, as they are unique and can be used to accurately identify a subscriber. CRM IDs are also a viable option, as they can be used to identify subscribers based on the data stored in the CRM. Reference: https://help.salesforce.com/articleView?id=mc_es_subscriber_key.htm&type=5