There is nothing bad about the ketogenic diet, but there’s nothing magical either. It doesn’t change the rules of thermodynamics or human physiology; to lose fat you must be in a calorie deficit, keto or not. Some people see tremendous results following the ketogenic diet, but it’s also very common for people to gain weight when starting.
Remember that fat has 9 calories, compared to the 4 calories in protein and carbs. By removing carbohydrates and drastically increasing your fat intake, it’s very easy to significantly increase your calorie intake and begin to store body fat. It can also be tough to keep the proper balance of fats; if you start adding bacon and butter to your meals, you might run into long-term health issues with your cholesterol. Finally, if you’re into any sort of athletics or performance, the ketogenic diet is not optimal for performance. You’ll be able to finish your workouts just fine, but you’ll never perform as well as you would with carbohydrates.