Calories in/calories out can fail in some extreme cases due to weird combination of macros. It happened to me. I tried going vegan for 6 months and I was not getting enough proteins. I was weighting my food and eating more calories than before, but I was still losing muscle and thus weight. For anyone wondering scientific studies have proven you need to eat a certain amount of protein to keep your muscle. It won't be optimal but you can substitute carbs and almost all fat for any other macro, but protein you need a certain quantity. So yes, there are some corner cases where calories in calories out does not work, but its not the problem fats have.