First I'd like to anwser this question, "is orthogonal and there's no reason it can't be implemented [in] IBus."
There are things that IBus cannot do or hard to do because of architecture difference between Fcitx. As for sunpinyin, there are several additional plugin to add more feature in Fcitx, which ibus simply cannot do similar thing. Even it want to, it will duplicates code and lost consistency between engines.
As for integration,
Support of IBus in gnome now is even currently not well itself. From my point of view, it should not be landed in gnome 3.6 unless it gets improved. The integration currently still have lots of problem which will break user experience. (At least for now).
BTW, I'm also trying to provide input method integration on different desktop, thus I maintain kimpanel in KDE Plasma, and for GNOME-Shell, which can work with both ibus,fcitx,scim.