Everyone's brain is different and substances may don't affect everyone in the same way. Like how some people get addicted to coffee or cigarettes and some don't, some people get the benefits of Modafinil without the side effects and some don't. Cognitive disorders are one way to categorize this but it can be more complex than that. People with OCD or GAD are an extreme example, there's a lot of different personality types and brain structures out there, that's the "neural fauna" I'm talking about.