I'm hearing "losing weight is hard" a lot around now as a way diverting a failure of efficacy into an issue of difficulty. Saying a thing doesn't work is not the same as saying its hard to achieve, the two are not the same and should not be conflated. Dieting fails whether it feels hard or not, it is dieting's dominant principle that fails to work- that of restricting intake and increasing use of energy that fails.
It doesn't fail so much because it cannot induce weight loss, it fails because it provokes defense mechanisms that come into operation the moment you create a sustained calorie deficit. Some are so sensitive that the intention of doing that is enough to provoke a titan of a reaction which they cannot stand.
The hardness of dieting varies, but on the whole it tends to be quite a challenge that feeling is those defences acting to thwart the effort. Even that feeling is absent, they still operate, that's why many who find dieting easy still end up re-gain, if they can even sustain the effort in the first place.
It's the same for other examples, drugs, jaw wiring, surgery, exercise etc., they all fail to make people thin for the same reasons slimming doesn't slim.
Those claiming that diets don't fail they are just wreally hard are ignoring that even if that was true (it isn't) that would mean it would succeed with very few people. The whole the harder something is, the fewer are likely to succeed at it, that's plain common sense.
If you want something to be viable over a large scale it should be doable for most people, or don't expect it to be effective.
Theoretical in this case as the issue is structural, the hardness of dieting is not why it fails, it is its failure in action. Typical dieting mentality, will say anything to preserve it.