Moderated calorie consumption provides energy for the body and vegetarians and vegans do tend to consume less(1) [though again, calories aren't an end-all health strategy]. As cheese is used as a protein source in place of meat for many vegetarians, "ounce-for-ounce, cheese contains only about 80% of the protein found in chicken, but [has] nearly 2.5 times the calories" and therefore is not as protein-efficient(1). Many plant-based proteins tend not to have all nine essential amino acids in them, so combining different sources tends to round out the profile more(2). [Having milk is a food-culture mandate and yet] a cup of almond milk tends to have a gram of protein and a bunch of added sugar compared to 8 grams of protein in cow's milk. Plant foods may include iron but also need Vitamin C to absorb; omega-3 fatty acids and Vitamin B12 are also deficiencies vegetarians are prone to(1).
Vegetarian, vegan, or meat-inclusive diets can be healthy or not. It all depends on the whole context of what's in them. I actually like veggie curry better than a steak by itself so, getting in as good a mix of nutrients as possible can be fun. Culture teaches us that certain foods have to go together in a manner often arbitrary of healthiness, so letting go of cultural food mandates will need to happen at some point as well.
Personal Product Favorite:
Complete, complete plant-based meals
Sources:
1. Link, Rachael. "12 Mistakes to Avoid on a Vegetarian or Vegan Diet." Healthline. Healthline Media. 28 July 2017. Web. 21 Sept. 2020. https://www.healthline.com/nutrition/vegetarian-and-vegan-mistakes.
2. "What To Look for When Choosing a Plant-Based Shake." Isagenix Health. Isagenix Worldwide, Inc. 25 Aug. 2020. Web. 21 Sept. 2020.
3. "Plant-Based Protein Explained." Isagenix Health. Isagenix Worldwide, Inc. 28 Jul. 2020. Web. 21 Sept. 2020.