The start of a keto diet can be difficult, simply because many people feel deprived when cutting out a lot of junk but after a few weeks, the cravings subside and you find that appetite is reduced making very easy to maintain, some people I know actually have to consciously make themselves eat enough, especially if they are looking to build muscle.
While it is very low carb, those carbs should be from healthy vegetables, preferably greens with plenty of fibre, If your filling your carb limit with non nutritious foods the you will have a lack of vitamins just like any other calorie controlled diet, it is poor food choices not the diet itself.
A high carb diet will have the body hold a lot of water weight, so the big loss at the start of keto is water weight and not fat, so if somebody only tries keto for a short time then that water weight will come back as well giving the impression of a yoyo, once the body is actually adapted to keto then the fat you loose will not suddenly yoyo back if you stop keto, just the water weight you loose in the first couple of weeks.
The one thing you do need to keep on top of for keto is electrolytes, with the loss of water you flush out a lot of salts so if you also try for a low salt diet at the same time then cramping can be an issue along with the fatigue and lack of energy people new to keto complain of. The two main electrolytes are sodium and magnesium, with magnesium possibly needing supplementation.
When I saw an endocrinologist during my pregnancy (gestational diabetes) she pointed out the she recommends a keto diet to most of her diabetic patients, although I am not diabetic, I do track my blood sugar reading due to family history of late onset type 1.