A vegan diet can make it a little more challenging to obtain adequate amounts of some key micronutrients, such as iron, calcium, magnesium, zinc, and vitamin B12.
However, this can easily be overcome by thoughtful food selection (or supplementation), and a vegan or vegetarian diet can easily be adopted without unfavorably impacting health, body composition, or performance.
There is ongoing debate and argument about whether or not a well-designed vegetarian diet is inherently more healthful than a well-designed omnivorous diet, but a plant-based diet with plenty of protein and no micronutrient insufficiencies can be viewed as an excellent diet option that is neither worse nor dramatically more advantageous than a well-designed omnivorous diet.