I've found that using the mobile url is useful.

Remember you can change the date (in the URL) to any date in the future.

Of course, out of the 24 hours window, it will show only those pax upgraded by confirmed SWU or miles.
(Not really 24 hours window for check-ins, since a connecting flight may allow you to check-in in a flight out of that window)

Today is Wednesday, but I can see my confirmed upgrade on next Saturday flight. (I'm CAM/D)
Of course nobody has checked-in yet.
https://mobile.united.com/FlightStat...Option=Upgrade

To use that, check for your flight, on any date, and tehn manually change the date in the URL
That will provide you information about who has already been upgraded (if any).

Regarding the usefulness of Flight Status information.
Confirmed upgrades sou see AFTER CHECK IN are those not upgraded with a voucher (or miles)
Remember also that all upgrades that have not been confirmed 24hs prior to the departure time will be confirmed at the gate.