here's a good link to the advantages and disadvantages of
ketosis
the biggest disadvantage i can see is the risk of muscle catabolism (loss) from potentially low glycogen levels
i don't like diets like this that promote extreme (and generally short-term) changes to what you eat, rather for me its about adopting the correct 'lifestyle' because this is something you can do long-term.
people constantly seem to chase that elusive 'lose fat quick scheme' and the damage they can potentially do to themselves physically or psychologically usually outweigh the claimed benefits.
patience is much healthier.
i discuss these concepts and put some links to low-carb ideas in this
thread