The limitations of connecting Marketing Cloud Account Engagement to a Salesforce Sandbox account are: if connected to a sandbox first, when you create your production connector, all prospects will not automatically queue to sync, and Marketing Cloud Account Engagement can’t “wipe” data, so you will need to manually delete any prospects and data that is brought into your Marketing Cloud Account Engagement account from the sandbox connector before enabling the connector for your production instance. A Salesforce Sandbox account is a copy of your production environment that you can use for testing and development purposes. Marketing Cloud Account Engagement Sandboxes are test versions of a Marketing Cloud Account Engagement Business Unit that you can provision within a Salesforce Sandbox. However, Marketing Cloud Account Engagement Sandboxes have some limitations that you need to be aware of before using them. One limitation is that if you connect Marketing Cloud Account Engagement to a Salesforce Sandbox first, and then create a production connector, the prospects in your Marketing Cloud Account Engagement account will not automatically sync with the production environment. You will need to manually sync them or use an automation tool to do so. Another limitation is that Marketing Cloud Account Engagement cannot erase the data that is brought into your Marketing Cloud Account Engagement account from the sandbox connector, such as prospects, fields, and assets. You will need to manually delete them before enabling the connector for your production instance, or else you will end up with duplicate or unwanted data in your Marketing Cloud Account Engagement account1112 References: 11: Marketing Cloud Account Engagement Sandboxes: What They Can, and Can’t Do12: Use Marketing Cloud Account Engagement Sandboxes