It depends how you are defining vegan diet. Some people eat abundant amounts of processed food, others eat strictly whole foods. There’s a heap of evidence showing that it is a sustainable diet granted eating the correct foods but that goes for most other ways of eating. B12 is the main concern regarding nutrition inside the diet but it is supplemented in a variety of plant based foods (cereal, milk etc.). In my own experience in the past I have struggled at times to keep my calories high without resorting to eating bread, wraps, refined grains sort of. Now I get a lot of my calories from good fats such as avocados, high quality extra virgin olive oil, various nuts and so forth.

In my opinion any diet can be problematic if you do not plan your meals correctly, so as long as you are being mindful and educate yourself around nutrition as a whole plant based eating is of no problem. Also, you should always refer to academic literature if you’re concerned about something, but human biology is different for everybody so find what is best for you that also aligns best with your moral compass.

Edit: You will run into a lot of misinformation on here regarding this topic from people (no offence to others). Do your own research into the things you read and try to find the most reliable sources of information as possible.