It's mostly the unhealthiness of the diet. Sure people lose weight on it, if followed correctly. Hell, my mom has lost weight on it.
But- she was eating things like cream cheese wrapped in slices of ham, the meat and cheese off of pizzas, the meat out of hamburgers... It seemed almost like she barely ever ate grains (it does allow some in a phase or two. I forget how it works)! It's losing weight, but not exactly healthy.
So she went to the doctor about losing weight. Our doctor told her, "If it's white, don't bite." No white bread, potatoes, rice, etc. So now she gets her carbs with red potatoes, wheat bread, brown rice, things like that. She still seems brainwashed about the carb thing though. She told me the other day, "You should watch your carb intake..." ...WHAT? First of all, it's not like I gained weight or anything, because I haven't. Second- No, my diet is healthy enough! I get a pretty good intake of all things I need, I mean, I don't eat carbs from grains in every meal, but I eat them as I need them.
This diet upsets me. Eating the way I do hasn't made me gain weight yet.