I think the main drawback to the vegan lifestyle is that it's common that the concern for the animals and environment causes you to reject any study that suggests it's not the best way to go long term for everyone. It becomes more of an emotional issue rather than a Science based thought process. You see what you want to see is the line that applies to many. If a study "proves" what you want to continue believing then it's a good study if it suggests you are currently misinformed you just ignore it and say it's a bad study.