I've met two fruitarians who claimed to be several years into it, both seemed healthy and one was very active and fit. I believe both of them but have no idea how that diet works.

I've heard that many carnivores start adding fruit back into their diets to deal with some health issues, but I don't recall what those were.

I don't know anyone who went vegan from carnivore, but I know lots of people whose health improved after going from meat heavy diets to vegetable and bean heavy diets. They mostly didn't stick with those diets, though, so I don't know much about long term ramifications. I'm also aware that there's a categorical difference between a meat-heavy American diet and an actual carnivore diet.