I also rely heavily on the Search all mailboxes function. I think this is an essential function and needs to be included.
A related issue - when composing a message in the previous Outlook, there was the option to change the account from which it is sent, even after you had composed the email - in the New Outlook, changing the account deletes any content I have written - I may as well just create a new email. Managing multiple mailboxes is a constant challenge and this is another function being removed in the New Outlook.
This is one of the features that I have been missing a lot and its definitely affecting the workflow. I do understand they are transitioning everything to a web-based system, but when this is done, some essential features need to be worked into the new one which I believe is crucial to most businesses.
Reading this thread I can see its been a year and none this has been adapted to the new outlook yet. It is sadly disappointing that they force us to the new outlook without ensuring all necessary features are migrated.