CICO is great if you want to lose a few pounds, and if an obese person sticks to it, they will lose weight. But while CICO is a great tool, followers of CICO as the one and only way frustrate me, as they turn weight loss into a moral struggle solely about willpower. To them, the only reason someone is fat is because they are weak willed and any desire for an "easy way" out through anything other than CICO is viewed as laziness. Yet, while most of these "easy ways" out are indeed fake, we do know that in several specific cases there is a root cause separate from willpower, such as thyroid disorders and PCOS.
In these cases, it is not simply a matter of the affected person having less willpower, but rather that they need greater willpower to use CICO to overcome their obesity than their non-obese counterparts. Note that in these cases CICO does still work, but once you shift the perspective from "these people have less willpower" to "something is requiring these people to have greater willpower to do the same thing" then a new question immediately arises: "what is that something?"
Considering we know such root causes are possible, is it so unreasonable that rather than 1/3 of the population having bad willpower, that instead they have some sort of invisible disorder that is the root cause? Before 1982, gastritis and peptic ulcers had unknown causes and were attributed to stress. These days, we understand there is a bacteria H Pylori that is the root cause that we can give antibiotics for and "magically" cure. My personal belief is that within 100 years we will find some sort of similar root cause for obesity and if we are lucky, we may similarly be able to find a "magic cure" for it too.
Meanwhile, as someone who is obese and is working on it, I absolutely do recommend CICO as the best tool we currently have. But it's strikingly similar to trying to get a homeless person to budget in that there are likely other things that need to be fixed first, we just don't know what they are.