Can the tab's currently edited document be associated with the tab beyond simple "title" metadata? For example, can I get an icon on the tab representing the tab's in-progress document that I can drag to drop sites like Slack or an email being composed?
What statuses can be visualized on the tabs out of the box? (Percentage complete? Ancillary status icons?)
How's accessibility going to work with tabs? What happens to the client area when you switch tabs? What gets read on the tabs when you cycle through them?
I suspect these Sets are not going to make MDI great again because they will be underspecified as a UX standard or will be denuded of any productivity features (mobile first), and thus will be abused by developers who resort to hacking to get the results they want.