With Nuclide, you can build for iOS, Android, and the web. Most IDEs only support one of these platforms well.
Although I am disappointed by the current speed of Atom, I have no doubt it will improve. I think it is the most exciting OSS IDE out there. We hope to slowly converge on JS+Flow as the main front-end language at Facebook, and it will be a huge win to build tooling in the same language as the product.