What does Atom bring to the party that Sublime Text 3 doesn't?

All I can of (off the top of my head) is slower and chews more memory but is open source.

Everyone develops packages for it, and they're much more actively supported than ST3 packages. It's really the ecosystem.

My experience is that the overall Sublime packages are much more stable and do not need so much rework and maintenance. I cannot even count how many times Atom packages failed (maybe it's also just because I cannot see fancy failing error messages of packages in Sublime).

I think that's really a side effect of it being free. (And I wonder if VSCode will eat that ecosystem...)

Do you have examples where the Atom/VSCode package is much better maintained than the ST3 one?

The interface for one. OOTB Sublime Text can't install packages, for example. Once you install Package Manager, discovery is terrible; you have to know the name of the package you want beforehand. On Atom/VSCode you can easily browse all packages and install them from the editor itself.

Visual Studio Code?