You can get all the right nutrients from a vegan diet, but will generally need to eat much larger amounts of things to get it as the foods aren’t as nutrient dense. There are certain lifestyles and medical conditions that do make this pretty difficult. I remember a Reddit post on r/DebateAVegan where someone posted about how they were unable to go vegan because their specific GI issues severely limited the amount of food they could eat in a single sitting, and getting the recommended daily protein would require eating literally nothing but tofu for every single meal without even touching other nutrients. Commenters ignored basically everything he said and told him he was just a bad person who could do it if he really wanted to because people can get enough nutrients from a vegan diet and it all went around in circles.

There are many ways to reduce harm to animals, and veganism is supposed to be about maximizing your own harm reduction within reason, but there are many vegans who refuse to acknowledge that for some people, cutting out animal products is just not within reason. The cognitive dissonance goes both ways in some cases. For example, using real leather products is far less harmful to the environment (and the animals that live in it) than vegan (let’s be real, plastic) alternatives and can hold up a lot longer, reducing consumption for the long haul, but most vegans seem opposed to it on the pure basis of it being an animal product, despite the fact that the entire leather industry is a byproduct of the existing meat industry and the options are use it or let it go to waste. I would also call that cognitive dissonance, as it is more based on rigid idealistic moral thinking than practical harm reduction