You may notice that there may be a block in SFDC that stops syncing unsubscribes from Marketo to SFDC. After fixing the block in SFDC the new unsubscribes will sync as expected. Here is how to get all the unsubscribes that happened before this fix to sync into SFDC.
Unsubscribed = true
Change data value "unsubscribed" -> False
Change data value "unsubscribed" -> True
That would take everyone who is currently unsubscribed, switch to false and then back to true. That should cause the true value to sync up to SFDC for all of those records. However, make sure you do this at a time when you are absolutely positive there are no email campaigns going out.