Vegan diets are very popular today especially with younger people. The diet is gaining momentum at an amazing rate, due to the fact that people feel it is a way to be more health conscious and protect the environment and animals.
Here is my own personal opinion on it. I feel it is very hard for people to get adequate protein intake due to the fact that most animal products are loaded with protein. Looking at it from a bodybuilding/physique standpoint, a person needs correct protein intake to fuel new muscle growth as well as strong workouts. Vegans diets often contain many processed carbohydrates due to the fact that many of times these carbs do not contain any "animal bi-products" but refined carbohydrates are never a healthy alternative. Processed carbs and bleached grain products spike insulin and raise blood glucose levels. This can increase fat storage through hormone response as well as increase cancer risks.
I approach vegan diets as I approach any other diet. It comes down to macronutrient balance and getting the right amount of protein, carbs, and fats. You can build muscle and gain strength with any type of diet (vegan, keto, paleo, atkins, ect...) if your protein is high enough and you are in a calorie surplus. Same goes for fat loss/weight loss. You can lose fat with any diet (vegan, keto, paleo, atkins, ect...) if the calories are restricted enough.
Another problem with vegan diets is that it is a struggle to get enough fat in the diet each day. The body needs two things to function. The first is essential fatty acids for proper hormone function which is hard to get with vegan diets. The second is essential amino acids (protein) to keep strength and muscle. There is NO such thing as an essential carbohydrate, which makes up the entire backbone of vegan diets. No, it is not impossible to get fat in a vegan diet. There is many options like nuts, avocados, and oils. Same goes for protein, there is many plant based protein options like nuts and beans. However, it is tough to always have access to these foods at all times.
I feel that vegan is an excellent diet if that individual is also tracking their macros to get proper protein and fat each day (which goes along with any diet) with whole foods such as vegetables as the backbone of the diet. This will help them get closer to their fitness and physique goals. However, if that individual is using the vegan diet as an excuse to over indulge in excess process carbohydrates in copious amounts just because it's considered "vegan" than that is where problems will occur. It comes down to the macros and the numbers always. Try it out for yourself and see how you respond!