I've googled "Error: MigrationPermanentException: Active Directory property 'homeMDB' is not writeable on recipient" and have tried all the fixes found. Most people have been able to solve the issue by inheriting permissions or adding the Exchange Trusted Subsystem permission. The users I'm having a problem with have all of these permissions enabled and inherited, but the mailboxes still don't go anywhere. The interesting thing is that the migration batch completes without error or failure now, but it says "0" mailboxes were migrated or synced. Also, when checking on the status in ECP it doesn't show the mailbox as syncing. Then it just completes without error.
Not sure what else I can do here, so looking for other answers.