people's unwillingness to cooperate != difficulty of treatment
Asthma also isn't the only condition that is hard to treat in morbidly obese people. For many conditions it will be too late to start paying attention to one's calorie intake, but asthma is one that is survivable in the meantime. Severe discomfort and possible death tends to be a good motivator.
Researchers generally analyze intervention success by intention to treat.
If a treatment is telling people to "start paying attention to one's calorie intake," and that doesn't have the desired effect, whatever the reason, I think it's fair to say that intervention isn't useful.
It's very easy to fill out a lottery ticket, but success rate is very low.
Success is a boolean condition that often first needs to be defined, while difficulty is a spectrum and a more rigid concept. Really the only complication is subjective vs. objective difficulty (what is objectively difficult may be subjectively easy to someone practiced).
Difficult things require hard skills. Reducing someone's calorie doesn't require any hard skill that I am aware of.
With which you mean the process causes discomfort? Probably less so than being asthmatic and/or a host of other things, but yes.
Other than that there can be some psychological issues that make self-controlled treatment subjectively hard or impossible. But then we'd have found another condition that would need to be treated first, getting us back to where we started: Somebody running a study on the efficacy of a diet probably would want to preclude people psychologically unable to stick to it...