I can tell you this from personal experience:
I was a vegan for 2 years. I did my research, I got every nutrient I needed, I certainly wasn’t eating fake mac and cheese and gross prepackaged “meat analogs” or tofu, either. Nothing but properly combined quinoa, brown rice, sprouted grain bread, beans, tempeh, and nuts, plus my supplements and mass quantities of vegetables and fruit.
That said, I an an estrogen dominant endomorph, so I started gaining weight, slowly but surely. I’m sensitive to carbs, and I had to eat them all day long just to try and get in the FDA recommended amount of protein.
Well, I got fed up with that crap, and started doing research. I needed to lift weights because I have tendonitis in my knees and pronation problems when jogging, so endurance athletics just aren’t in the cards for me. When I started lifting, I realized my calorie intake was far too low, and I didn’t want to choke down the disgusting hemp based protein powders for the protein I needed, so I decided to start eating fish again.
Best decision I ever made. Nowadays, I eat chicken and turkey too, but I stick to organic free range meats. I still avoid dairy except for organic greek yogurt.
In my case personally, I could not stay lean on a vegan diet. Nutritionally, it’s sound, but if you have any goals as far as your physique is concerned, it may not be a great choice for anyone sensitive to large amounts of carbohydrates.