I have friends who are vegan and they have a very good diet.
The problem that comes along with veganism is the potential for extremism. People become vegetarians for many reasons, including choices about animal welfare, but not solely and this often moderates their attitudes towards those who are not vegetarian.
However, in my experience the vast majority of vegans are driven by their ethical viewpoint as much as dietary choices. This means that there is always a "better" vegan. There is always someone out there who is more purist than another and this leads to polarisation of opinion along with viewing anyone who doesn't agree, including other vegans, as wrong.
The self-righteous, preachy and confrontational nature of some vegans is exactly the same as the behaviour I have seen time and time again by those who view themselves as spiritual supremos in the Jehovah's Witness faith.