Is it true that a low-fat diet is not effective in long term weight loss?

I do not think so , The mecanism of weight loss is burning the calories you have ate and when you eat fats which have more calories (9 cal\\g) you wil increase your weight .

so you should reduce your fats intake .

In my opinion, the diet would only be as ineffective as the person on it. You may get to a point where you stop dropping "weight"but cannot possibly reduce your fat intake any lower if you intend to stay healthy. That is when you either have to reduce carb intake ,increase activity levels or do both if you expect to continue dropping  body fat..If the person dieting cannot do this the diet becomes ineffective PAST THAT POINT (weight/bodyfat) hence my opening statement ,unless you enlist the help of thermogenics or some other metabolism boosting aid which again some people are incapable of doing whether for health,religious or moral reasons. Of course with different people come different genetic makeups,metabolisms,mindsets and all that so the same rules do not apply to all. I mean,can you imagine an Eskimo on a low fat diet?