Mike's `react-rethinkdb` project is really cool, but it's based on exposing the entire ReQL language to the browser. We thought this approach wouldn't work well as an industrial-strength product -- ReQL is designed to be accessed from the backend, and exposing a full query language to the browser introduces a number of challenges. Security is really hard, learning curve is steep, there are DoS challenges, the protocol is too complex to write platform-specific extensions, etc.

On the other hand, Horizon exposes a simple API/protocol that's easy to learn and easy to secure. If you need more functionality, you can import Horizon as a Node.js module and start writing backend code that accesses RethinkDB directly. The learning curve is much easier, it provides an upgrade path for sophisticated apps, and feels like a more robust product.

(I don't mean to disparage `react-rethinkdb`, I think the project is awesome, it's just a philosophical difference in approach)