react specific
That's a temporary limitation that won't exist forever. We went with React first to get a feel for the API and seeing if people like it, but we will probably expand in the future. (you heard it here first, but don't tell this to anybody yet)
a lot of repetition: not-DRY + every component has its own specific CSS
The whole point of having components in the first place is reusing them throughout your application. If you find yourself having a lot of repetition of CSS in your components, you might want to think about how you're structuring your component system, how you could split up big chunks of JSX into multiple, tiny components etc.
That being said, I love Tachyons and think it's great—what works for you works for you, don't change a running system if you don't need to!