how does it compare to Atom
It does different stuff. Atom (and similarly VSCode) are meant to be closer to IDEs than just text editors. You could theoretically write an addon/plugin/extension to do everything obsidian does (or everything that Ulysses does, etc)
Obsidian is just a markdown editor, but has additional features around like your library of docs, linking between them, inline picture stuff from the looks, backups of your notes, etc.