the basic pattern i have seen:

far in advance, JFK is routinely available for U

far in advance, EWR is rarely available for U

however, EWR often opens up 45-60 days in advance if the cabin is not heavily sold (ie, if the cabin is less than 50% booked). JFK often shuts U down by this point because people have claimed upgrades into the long-available U space.

so, if booking far in advance, JFK is more of the sUre thing. if you are looking less than a couple of months in advance, i feel like it is more of a crapshoot.

and FWIW i think your chances on JFK-SEA are not that bad. i would say 50% or better that you get the complimentary upgrade, depending on the day of travel.

YMMV.