I've literally got a science experiment in my own body that shows reducing calories in, without reducing the actual design of my meals, reduces my body mass.
I'm willing to accept that there are some minor irregularities and difficulties that make "Calories in == Calories out" not 100% accurate, but I'm betting the effect size is closer to +-10%, and therefore easily discarded for approximations, even though they are scientifically significant and could create a more accurate model.
There's already a lot of uncertainty when most people measure their calories (very few people actually weigh their food) and this just adds another layer of uncertainty. I have a feeling those all combine to make it inaccurate enough in practice for some people to claim the CICO model doesn't work.
By not being hungry and unsatisfied you'll then stop overeating (surprise!).
"My diet is OK, I just eat too much" is all wrong: there is a complex relation between caloric intake, which foods are eaten, hunger, satisfaction, energy, mood etc.
Many fad diets "work" even if they are not grounded in any scientific fact and are even unhealthy in the long term (low fat, low carb, keto, gluten-free, all-meat).
They artificially restrict the variety of food one person can eat and this indirectly encourages people to eat less. And when people stop overeating they feel better and believe the fad diet is sound.
There were even a diet where you can only eat foods in a given meal from the same group... by color. Same trick.
Bracing for all the downvotes...
Just above you said a diet needs to be nutritionally complete. Low carb, keto, gluten free, hell even low fat can be nutritionally complete and satisfying, though the latter one will not feel really good in the long term.
