It guarantees works for everyone. I lost all my excess weight in just 2 months, first time ever in my life going on a diet. And as a physicist I was convinced from the start that a caloric deficit MUST work. It did.

Yes, what I can say it will be harder for some persons than others. The hormonal environment in the body can not be disregarded and can be either disrupted or genetically different. STILL, restricting your eating habit (caloric deficit), will.always.work.