Is there a way to hide synchronized data extension from SFMC users? I am currently on a multiorg with 5 biz units and have them connected to 1 SF org. With the Sys admin integration user in SF, the individual BU's have access to the data across CRM. I have tried to limit the access of the SF API user via permissions but it's causing an error in JB. Now i'm trying to find a way to hide the synchronized DEs. Any advise would be helpful.
When connecting to the same business unit, as you are doing, you have two options - either you limit access via the integration user in Salesforce and accept you cannot use Salesforce Journey Builder Activities - or you provide full access to the data and can use the aforementioned Activities.
This is driven by the fact that Salesforce Journey Builder activities require Author Apex permissions which requires Modify All Data permission (overriding any data access permissions)
Additionally, even if you could hide synchronized data extensions by using roles in SFMC, the records which are sent to Journey Builder would not be filtered by your record access since the events are being triggered by process builder, and therefore execute so long as the basic criteria you define in Journey Builder pass.
Please note, this scenario is not explicitly supported so you will be on your own when it comes to support:(