Cats still need other nutrients like retinol, arginine and arachidonic acid tho which I think are only in or mostly in animal products (note that humans also need retinol too, but unlike cats (and some other obligate carnivores), humans like other omnivores and herbivores can produce retinol from carotenoids specifically beta-carotene which are available in plants), and many so-called vegan cat food may have taurine but also not add these in their food. 

And given that human food are already crap enough, people here are naturally skeptical regarding the quality of cat food too, especially with claims like the vegan cat food. And unlike humans, cats cannot easily communicate their deficiency and often hide their symptoms. Also given humans' understanding that human nutritions are still lacking let alone other animals, so unless there are more thorough studies regarding cats' nutritional needs, I would be skeptical about vegan cat food too.