Following such a diet means you will be replacing carbs with foods rich in fat and protein, and if followed over an extended period of time this may have consequences. An intake of high fat foods is likely to increase your saturated fat intake which current UK government guidelines recommend that we limit to 30g for men and 20g for women. High levels of dietary protein are thought to be an issue if you have an underlying kidney condition. However, most ketogenic diets supply moderate rather than high levels of protein.
I eat a diet that varies from ketogenic to simply very low carb, and I love it. After a lifetime of being overweight and unfit, I am now, in middle age, in the best shape of my life. I weigh less and wear a smaller size than I did in my teens. I am also stronger and fitter than ever. In addition, my physician was "keep[ing] an eye on" a number of health issues of the type that most people assume are the inevitable result of ageing: Elevated blood lipids; elevated blood pressure; elevated blood sugar. I attribute a great deal of my success to the fact that I was motivated this time much more by health than vanity. They wanted me to go on statins, and I just refused. I applied myself in earnest to a low-carb way of eating, and in the course of my research I learned about ketogenic diets. I normally eat about 75 grams a day of fat; when I am riding regularly, that's often 100 or more; during long-distance rides, 150+.