a good diet simply means you don't shoot yourself in the foot every day.

carnivore diet when supplemented with vitamin C is a reasonably good diet.

when combined with fasting or calorie counting is an excellent diet.

but without a source of Vitamin C is dangerous at best.

yes Carnivore gurus will say they never take Vitamin C and are fine - but diet gurus are notorious for lying - i taught most of them ( Shawn Baker etc. ) how to eat correctly and they most likely do what i told them, but tell you not to take Vitamin C because frankly if you're stupid enough to trust them then you deserve to die.