The Atom Reason plugin uses Nuclide's system for error rendering (in the diagnostics bar), and type hints. But the actual logic for integrating with Merlin has been completely rewritten in Reason itself, and compiled from Reason into JS to run as an Atom plugin. So there are still merlin features missing because the Atom plugin is relatively young, but if you want to help us implement the missing features, it can be a fun way to try Reason itself (compiling to JS).