If they are releasing this year, then it must be down to completing levels now. I am sure the writing part was done long back, prbly one year after UC2, all writing must have been over. We do know that cutscenes were being mocapped earlier this year. They are reaching completion, for sure.
But , yes, it is sad to see her go. People ARE the games, if ppl leave, so does the charisma of the game. After all, everyone could feel UC3 was not donw by the same team as UC2. I think the lead game designer had also quit after UC2 to teach in some college. THts two down from the creative front on UC.