The common ingredients and their actual effect sizes
Caffeine can modestly increase metabolic rate (roughly 3-11%) and green tea catechins have a small additional thermogenic effect — real, measurable, but far smaller than marketing implies, and nowhere near enough to replace diet and exercise.
Why the results in ads look so dramatic
Marketing photos and testimonials showcase outlier results, often combined with an aggressive diet and exercise program the supplement gets credited for — the pill is rarely doing the heavy lifting it's marketed as doing.
Where genuine risk exists
Some fat burner products, particularly those from unregulated or overseas sources, have been found to contain undisclosed stimulants or banned substances — a real safety concern beyond simply not working as well as advertised.
A more honest framing
At best, a well-formulated fat burner might provide a very small edge on top of an already solid diet and training program — not a replacement for either, and rarely worth the cost relative to the modest effect.