When is the last time things didn't change that had impacts they couldn't foresee and include in their score?
It is like predicting what a healthy Panther team might do. Then bash the prediction because they weren't calculating the outcome with 3rd string offensive lineman playing most of the year.
CBO not being accurate doesn't mean it therefore isn't valuable. CBO can tell you what likely is a realistic outcome if Johnny Manziel is your QB or Tom Brady but variables they can't predict will make or inaccurate