Using New Outlook on Windows for personal email, calendar, and contact management
I'm not sure you read this correctly (or MS has changed the spec!). Currently says will be possible to add Shared Mailboxes to New Outlook. That is not the same thing as unifying personal mailboxes.
To add salt to wound, I just found that you can't paste from one account to another (I store all my Contacts in one account, but can't paste a phone number from there to an email being written from another account). New Outlook really should be FOC as its feature set is not complete.