One of the main remaining functionality to be implemented is good support for large files. Looking at this issue , it seems Atom team is making some progress but there are still some problems to be tackled.
In 0.208.0 (released 7 days ago) they mentioned in the changelog Atom now opens files larger than 2MB with syntax highlighting, soft wrap, and folds disabled. We'll work on raising the limits with these features enabled moving forward. Little bit disappointed at the progress as you could open large file with these features disabled long time ago through a package "view-tail-large-files".
Just updated to 0.209.0 and using ember.js (1.9 MB) to test. Editing/scrolling has some delays but it's better than previous versions.
Good luck Atom team!