The page lists some of the known issues with the unity session after the Ubuntu Desktop move to GNOME
GNOME (and others desktops) have been moving to libinput but Unity didn't and installing the old driver leads to non working mouse settings under GNOME.
* The default GNOME applications might look less well integrated
Ubuntu was carrying non trivial patches to some of its applications (nautilus, totem, ...) to look better under Unity, in particular restoring the use of a classic menubar and avoiding client side decorations when being used under an unity session. To reduce the maintenance work those are being removed.
* No online accounts
unity-control-center and the unity dash are integrated with Ubuntu Online Account which is going to be removed from the Ubuntu archive since it depends on components that are unmaintained now. Tracked on Trello: https://trello.com/c/2cMgendl
* Shared GSettings keys
Some settings are shared between the sessions and needs to be separated Tracked on Trello: https://trello.com/c/2uAVLGcb
* deja-dup settings panel
deja-dup is building a panel for unity-control-center which creates a depends on that component which is being moved to universe -> new binary for the panel is resolving the issue
* unity launcher integration
some applications are using libunity to integrate with the unity launcher (nautilus to display a progress bar on copies for example, deja-dup, update-manager) which forces us to keep that source in main
to integrate with the unity indicator we build some code with libappindicator (update-notifier, vino, nmapplet, ...) which forces us to keep libindicator/libdbusmenu in main