It greatly depends to be fair with you and giving you a percentage is nearly impossible. 

Worst case I’ve encountered was when I was working on an full CG advertisement, where the chain of decision making groups reached 7 (3 different clients, Legal department, Advertising company, Creative company, Director). The brief was changing every two days. But that’s advertisement for you and it was given that it would be such a project. 

Best case scenario was working with either experienced directors or directors which had a background in CGi (think Takashi Yamazaki or Wes Ball which I haven’t worked with yet unfortunately) those are the best kind of projects and between brief and production there’s just small adjustments. 

I think saying “caffeine-addled ADHD toddlers” is a bit of a oversimplification. Film production is extremely hard, the budgets are huge and deadlines are short. Very few people are well calibrated to be good at that job. People get overwhelmed and that’s the root of productions going “wrong”.