one - you cant go no carb, so it requires very accurate moderation which is something people tend to be bad at
two - the cheaper sources of protein and fat, which you now must eat more of, are often high in omega-6 instead of omega-3, or have the wrong balance of saturated vs unsaturated fats, not enough amino acids, etc
three - if you go for Keto, getting hit by points 1 and 2 together can easily create a health risk. Too much protein kicks you out of ketosis as well. You might chronically knock yourself out of keto without realizing it and eat a bunch of foods that you really only get away with eating by being in
So even though a proper keto diet, for example, might be perfectly fine. Recommending a keto diet to the general public very well might kill some people who historically have a bad track record for moderating their food intake