I would advocate moderation in just about everything. Ketogenic diets have proven helpful to people with uncontrolled epilepsy and may be of benefit to epileptics in general, to victims of stroke and other forms of brain injury and possibly cancer. They come with other effects that may not be worth the discomfort or unintended risks to healthy people. This includes kidney stones and, in women and girls, amenorrhea. Amenorrhea is associated with bone loss, increasing risk of osteoporosis, and indicates problems in hormone balance. Supplementation with calcium may help with this, but then again, it might not. Physiology can be quite complex, and consumption does not necessarily indicate absorption is occurring. It is quite possible that something that causes hormonal imbalance in women may also cause hormonal imbalance in men. Playing with one’s physiology, when we don’t know the unintended outcomes of such play, may be inadvisable, especially long-term.
I have been a vegetarian for 30 years and an athlete all my life. I have excellent bone density, low blood pressure, low resting heart rate, low fasting blood sugar, excellent blood lipid profile and great strength to body weight ratio. I'm happy and healthy, even though I thoroughly understand I represent but a single data point. I do get a lot of advice from well-meaning friends that I need to eat meat and would be much healthier if I followed a ketogenic diet. Vegetarians, as described in some current diet books, are weak and "skinny-fat". I will leave it at this: The "optimal diet" for one person may not be the "optimal diet" for another, nor the optimal diet for all conditions. Humans are omnivores and physiologically polymorphic and adaptable. This has given us tremendous abilitity to survive in wildly different climes and environments, and has made us such a resilient species. It is also quite possible, that our individual resilience allows our bodies to adjust to dietary and environmental change.
The current state of research indicates no harm though, from short-term ketogenic diets. I have not found much in the way of research on the risks and benefits of long-term ketogenic diets in healthy adults. Anyone who wishes to add a citation, please feel free.
Kossoff, E., Zupec-Kania, B., & Rho, J. (2009). Ketogenic Diets: An Update for Child Neurologists Journal of Child Neurology, 24 (8), 979-988 DOI: 10.1177/0883073809337162
Bergqvist AG, Chee CM, Lutchka L, Rychik J, & Stallings VA (2003). Selenium deficiency associated with cardiomyopathy: a complication of the ketogenic diet. Epilepsia, 44 (4), 618-20 PMID: 12681013
Brinkworth GD, Buckley JD, Noakes M, Clifton PM, & Wilson CJ (2009). Long-term effects of a very low-carbohydrate diet and a low-fat diet on mood and cognitive function. Archives of internal medicine, 169 (20), 1873-80 PMID: 19901139