There's such a thing as too much protein. Long before the Atkins popularized keto diets, my dad once told me he tried a high-protein diet that I think he said induced kidney damage. I think I once read the normal recommendation is to get 15% of your calories from protein, but it's been shown safe to go up to 30%. Don't quote me on that: make sure you're following advice from a high-quality, trusted source.
The main idea behind keto is to replace carbs with fats. Ketosis is induced when your carb intake falls below a critical threshold. A lot of people do seem to have trouble maintaining ketosis for extended periods of time. And when you go off it, the weight rebound can be quite rapid.
On a nutrition label, insoluble fiber is counted as a carb, but it's not absorbed. I think a proper keto diet contains a lot of fiber. Just goes to show that not all carbs are equal.