I just don't think his personality translated to head coach. I think he's a pretty ok OC.
He was WR coach/ST coach/assisstant HC at MSU under JLS, and he was fine.
He's only been an FBS OC for 5 seasons. 1 at Fresno, where they were #38 in offense, but it's hard to judge anything off a one year snapshot. Then he was at Bama for 4 years. Again, fine, but how much under Saban is Bama fine with whoever his coordinators are?
I think it also all comes down to how much control Harbaugh is willing to give him. If it's still Harbaugh running the offense, it doesn't matter who the OC is.