A healthy vegan lifestyle gets plenty of variety with fresh veg, legumes, veg, nuts & seeds as part of daily intake. However some people come at a vegan lifestyle beset with ideas and in some cases chronic conditions that distort the way they go about their lifestyle.
For instance some people may go on a water fast, indeed I did a water fast once for two days as a lead up to the beginning of my recovery. But some people go on a water fast regularly, and that in my mind is plain stupid. It is silly because it is actually inviting health problems.
Not getting a regular B12 supplement seems also a mis-guided idea. I regularly consume seaweed which is a possibly source of B12, but that does not mean I do not take a this supplement.
Relying on juicing and smoothies day in day out can also encourage disaster. I am not saying following the ideals of a well planned smoothie approach for a period (say a week or even a month, if it is well planned) is a bad thing. But to consider smoothies as a lifestyle option is definitely a bad plan. Digestion begins in the mouth and smoothies bypass that entirely. Smoothies as a transition to eating normally is what is required, and then the occasional smoothie. If the rest of the diet is good, then even a daily smoothie.
Going fully raw can also be a mistake. Yes, some people seem to thrive on being fully raw, but I would argue that you need sufficient calories in your diet, and to do that is a lot easier with a cooked meal once a day.
This leads me onto the biggest mistake. Counting calories so that you are in serious calorie deficit. This is definitely the biggest mistake many people who go vegan make. Did you know that, no matter where you live in the world, no matter what diet you follow, that by and large you eat the same amount of food as everyone else!
What I am saying is fill your plate up so that it looks full at each and every meal. If you need/want to lose weight alter the balance of starchy/nonstarchy foods. Starchy foods include pulses & legumes, potatoes, rice & grains. Swapping some of them out for fruit & veg almost guarantees weight loss, despite having extra large plates of food at every meal. And you don't even need to exercise (though highly recommended)!
Making the change is easy for some. Some people seem to be able to drop eating meat and seriously increase their veg intake with ease. They feel healthier and everything goes swimmingly. Others start eating more veg and encounter problem after problem. It can take time for the gut biome to adapt to a more diverse diet. For most that change should take around three months, with many doing it in less than one month. For others it might take a year. If you are transitioning, take it slow, even take a year or more.
Finally misunderstanding and underestimting health issues is a definite thread. People come at veganism and think it is a kind of panacea. If people do have health issues then these can block a successful transition, and indeed their health issues may be exacerbated by a change towards a vegan diet. If you do have issues then geting these checked out by, ideally, a plant based doctor/nutritionist should be a prerequisite.