Overall this seems to be a very good diet for most people as far as fat loss is concerned. Some do deal with negative side effects while in ketosis but most people will find that although it's really hard the first two weeks, after that period their body begins to adapt and it gets much easier. Furthermore, one of the biggest benefits of being in ketosis is appetite blunting therefore it can actually be an ideal program for someone on a diet.
The only draw back you will see is for those who do have high activities or are involved in lots of sprinting type exercise. Although a few will find they feel fine, even have more energy on a high fat/moderate protein diet, most of the time carbohydrates are the best source of fuel for these activities. That isn't that big of a problem though, it just means that that person should instead look into doing a TKD instead of a CKD.
The first modern study of fasting as a treatment for epilepsy was in France in 1911. Twenty epilepsy patients of all ages were "detoxified" by consuming a low-calorie vegetarian diet, combined with periods of fasting and purging. Two benefited enormously, but most failed to maintain compliance with the imposed restrictions. The diet improved the patients' mental capabilities, in contrast to their medication, potassium bromide, which dulled the mind.