Explanation
A limitation that the consultant should consider when recommending Field-Level Encryption is that Field-Level Encryption doesn't support segmenting, filtering, or querying encrypted fields. Field-Level Encryption is a feature that allows marketers to encrypt sensitive data in Marketing Cloud using encryption keys and algorithms. However, encrypted fields cannot be used for segmenting subscribers using filters or queries, as they cannot be evaluated or compared with other values. References:
https://help.salesforce.com/s/articleView?id=sf.mc_overview_field_level_encryption.htm&type=5

When new records are continuously added via API and you want to inject them into a journey, you should use Contact Evaluation set to Evaluate new records only. This ensures that only the newly added (not existing) records are evaluated for entry into the journey.
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Exact Extracts from Salesforce Documentation:
Source: Salesforce Help - Contact Evaluation Settings in Journey Builder
# "When using a Data Extension Entry Source, you can choose to evaluate new records only, ensuring that only fresh entries are processed." Source: Salesforce Help - Data Extension Entry Source
# "Select 'Evaluate new records only' to inject new rows added to the data extension into the journey."
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Using personalization strings created by selecting the data extension can be used to include the warehouse ID in their tracking parameters so that the third-party system can identify the subscriber. Personalization strings are placeholders that can be inserted into emails or landing pages to display information from data extensions or lists, such as subscriber attributes or custom fields. Personalization strings can also be used as tracking parameters in links or image tags. References:
https://help.salesforce.com/s/articleView?id=sf.mc_es_create_personalization_strings_by_selecting_a_data_exte