> I think the problem is that "calories in, calories out" is not that easy since it's hard to measure the calories in and the calories out precisely.

You don't have to, precise calorie counting is a strawman argument against CICO. All the fat people I've ever known, which is most of my family and many of my friends, all eat way too much. I ate too much too, ever since I was a kid. Huge portion sizes and constantly grazing throughout the day without set meal times were the norm for me and those around me. After hearing CICO repeated often, I recognized these behaviors in myself and corrected them. I didn't need to count calories for any of it. Where I would once buy a pizza and devour it over the course of an evening, I now save half for lunch the next day. Anybody can do that if they choose to. By eating less, I've lost about 75 lbs in 5 years.