> But those who stick with regime, it works 100%

No. Even nearly a decade of intensive behavioral treatment has highly variable results.

Here's what the numbers tend to look like in practice:

  ~26% weighed more than at baseline
  ~23% lost >0% to <5%
  ~23% lost 5% to <10%
  ~16% lost 10% to <15%
  ~11% lost ≥15%

As you can see, results cannot be guaranteed.

> positive changes in life, quality, happiness etc are massive

This is generally true. Eating better and exercising will improve your life.

But will you lose weight? How much weight? Those are far more difficult questions.