On my Debian Unstable, it fails as below.
$ pkcon get-distro-upgrades
Getting upgrades [=========================]
Finished [=========================]
Fatal error: GetDistroUpgrades not supported by backend
So, I guess this is most probably some issue with the packagekit DBus
service coming up on your system, in which case you should see the
following, but not an endless wait.
$ pkcon get-distro-upgrades
Getting upgrades [=========================]
Finished [=========================]
Fatal error: Timeout was reached
If it's still reproducible, you can provide more information.