Protein is not the problem. Never has been. There are plenty of ways to get complete protein on a vegan diet.
But yes, there are often nutritional problems. My daughter tried really really hard to go vegan. She tried for a year and a half, doing all the right things, taking all the right supplements. She had to give it up. She started experiencing syncope (similar to a seizure) when falling asleep. Her LDL levels were too low. Her levels of various hormones were out of whack. Other essential nutrients were also low (we did blood testing to learn all of this). She now eats eggs, fish, and dairy, and takes fish oil. All her issues went away.
Some people have the genetic makeup to do well on a vegan diet. Many people do not, and no amount of supplementation will fix this.
Google for the many stories of ex-vegans whose experiences mirrored my daughter's. It's a difficult thing. I understand that many vegans are vegans for moral reasons, and it's very difficult to face the consequences of one's own genetic makeup. My daughter was angry at first, but has since come to peace with it.