Some plants are poisonous. Therefore veganism is unhealthy.
You don't need to eat every plant equally. Even if you have an allergy to a plant doesn't mean you need to take
Some animals, like the poison-dart frog ingest toxins that make it into their products. Famously, Abraham Lincoln's mother died from drinking the milk of a cow that had grazed on white snakeroot. So if anything, the existence of toxic plants should make one weary of eating animals that might ingest those plants without your knowledge.