I'm not too worried about how powerful anything is, I'm more worried about the endless number of questions people are going to ask about them and how they pair up / interact with other stuff.
Do I list all 3 books as playable options? Do I not list Daemon Hunters? When the older books reference GK's and say one thing, do we go with that or do we go with whatever (if anything) the new book says?
We want each player to operate out of a single codex if at all possible, but as I understand it, certain armies can "request" GK's to come aid them or something... I'm not sure if the rules governing that have changed at all, but if the old books are legit... who knows.