Well, I'm not gonna search and name names as it's not my intention to shame or offend anyone but I definitely recall reading a few posters admitting they're "back on" LCHF. Meaning they were on it, lost weight, went off it, re-gained weight. So according to the above concept of fad dieting, LCHFers may need to concede their diet is not the messiah. I do say may.
And you would in some cases be right. I refer to one of my own posts:
The problem I find with going absolutely super low carbs is that yoghurt, berries and nuts all have carbs in them, and eating all of them in one day probably means you are blowing past your carb limit.
If that matters to you.
I decided to not aim for super duper low carbs because I know it isn't realistic for me. There is no way I could eat less than 20g of carbs per day, so I don't even try. What I do instead is to eliminate unnecessary carbs (sugar in tea/coffee and sugar on top of breakfast cereal), and eat "healthier" carbs instead (only all bran for breakfast, only whole wheat low GI bread for lunch). And I avoid carbs for supper if possible.
I snack on nuts during the day.
The key point is that the lifestyle change - not diet - that each person can sustain is a personal thing. I know I cannot sustain an LCHF diet. Many have tried and failed. So their weight will yo-yo.
It is not the LCHF diet that is wrong, it is that it is wrong for some individuals who cannot maintain. Like myself.
Similarly, any diet that aims to make you lose a drastic amount of weight suffers from the same problem - can you keep it off afterwards? For most people, the answer is no, because they have no changed their lifestyles. Until you stop seeing food as a reward, you will put the weight back on again.
Now, the HCG diet has other issues, namely fraud and malpractice, but even if it worked exactly as advertised, I would still be against it. I do not think it is a permanent solution to weight loss.
Weight loss is hard because it takes consistent effort for the rest of your life. Not consistent effort for 3 months while you drop 30kg. The rest of your life. When you change your lifestyle, your weight starts to move towards a new equilibrium. Once that equilibrium is reached, your weight will be stable again. If you go on a crash diet, but then resume your bad eating habits, then your equilibrium weight is your old weight.
Can you go on a crash diet and then switch to a maintenance diet? You can, but I'm not convinced that most people will be able to keep it up because of the willpower required. You want a quick weight fix - there are no quick fixes.
How long does it take to get into shape? The rest of your life.