> Atom IDE UI is fast and lightweight by design. It extracts only the subset of the core UI features from Nuclide necessary to support Atom’s atom-languageclient library in displaying features supported by the language server protocol.

Source: https://nuclide.io/blog/2017/09/12/Introducing-Atom-IDE-UI/

So I guess the main differences are that it's more lightweight, and open-source.