I simply wish that he had some more experience as a coach before becoming a full fledged offensive coordinator.
I do believe that he has a good mind for the X's and O's, and that with time, he'll prove to be a good one, but right now? It seems to be too early.
Now, if / when I'm proven wrong, I'll be more than happy to eat crow, and it will be quite tasty indeed.
Tommy Rees has a veteran quarterback at the helm for 2020 (Book will be in his 3rd year as a starter, and knows Brian Kelly's spread offense quite thoroughly), along with a fine group of TE's, and a veteran offensive line at his disposal. I can only hope that the line stays a lot healthier than it did this year, though...
Having all of that in hand, he can afford to make more mistakes than someone with a poorly stocked larder.