Fad diets are notorious for their massive hype and total lack of substance. People could be forgiven for thinking that these diets are basically just confusing and contradictory. There have been high carbohydrate diets, low carbohydrate diets, fruit diets, high meat diets, vegetarian diets, etc.
In practice, any diet with a lack of balance can be totally inappropriate.
Diets are supposed to be based on the needs of the person, not the needs of the people selling the diet books.
The right diet is the one tailored to your personal condition.
Real, scientifically assessed diets aren't necessarily simple, and that's where fad diets are potentially dangerous. Eating a lot of a few types of food means you're probably not eating enough of anything else. That's not realistic. Humans are omnivorous, and the needs of the diet aren't really negotiable.
You must have the nutrients required for your health.
Everyone has different nutritional needs, and they vary a lot over time. If you're not eating well for a while, you actually need more of everything. If you haven't been eating protein, obviously you need to compensate. You may lack some nutrients, and have too many of others. The fact is that the different nutrients are supposed to be working together. Unless you have a good working mix of all of them, the diet is inadequate.
This is another area where fad diets are totally irrelevant, unbalanced, and basically useless.
The correct diet involves a complex analysis of your dietary intake.
If you can find a fad diet which actually matches your needs, it's like winning a lottery. Some diets are based on a scientific theory, like the Paleolithic diet, which is based on the theory that human dietary needs were pre-programmed in the early Stone Age.
It's worth taking these diets with a grain of salt. Basic foods, protein, mineral, and vitamins, with the right types of chemical balance, will always be reliable. Fad diets, which are unbalanced by definition, can only work where they provide the right nutrients for the right situation.