Lots of commentary on diets get down to measuring efficacy by how much supplementation you need to add to the diet. A common criticism of veganism is the fact that you must supplement for B-12 and that you must be careful with how varied your food selection is to avoid other deficiencies. The problem of deficiency is actually possible with any diet selection.
First and foremost, chasing a diet label as a substitute for critically looking at ones diet is a recipe for running into problems with deficiencies. This is especially true for diets that are more mainstreamed than others. You can see it most ridiculously in a Paleo diet devoid of vegetables or a vegan diet consisting of french fries and veggie patty burgers. Such diets may fit these labels but they are going to have holes in their nutrient profiles.
Less egregious than that is lack of variety in our diet from day to day. We all fall into the ruts of going to the foods we enjoy and are familiar with. Likewise we go with what is available and convenient to make or purchase. While this is infinitely better than the first example, this is is still going to create some nutrient holes. Certainly, there will be far fewer and probably the deficiencies won’t be totally absence but insufficient levels. The holes are there nonetheless.
Lastly, there are some nutrients that will be lacking regardless of how well you vary your respective diet. These sorts of nutrients are things like Vitamin B-12, for vegans as well as Vitamin D and K for most diets. Making sure that one gets adequate levels of these harder to find micronutrients is going to require a lot of proper planning and rotating of the diet. It’s not impossible but it’s difficult. The question then becomes what to do about it. The obvious answer to this is supplementation. By supplementing ones diet with vitamins or fortified food products one can eliminate all of these deficiencies relatively easily.
Of course, it would be wrong to take an overly reductionist approach to that advice. I can go to the extreme and eat nothing but hot dogs and french fries, take my supplements and therefore hit all the proper micronutrient levels. That’s is of course as absurd as my first examples of misplaced reliance on diet labels. It’s actually the same thing just with supplements. Instead think of supplements as augmenting an already healthy diet to make sure you don’t leave any gaps in your nutrient profiles. With an already healthy diet you could default to tackling this problem solely through diet tracking and profiling. However that’s really a lot of work that can be done with periodic moderated supplementation.