For me the biggest problem here is the high CPU usage when the app should be idling. That KILLS battery life. I would not accept this behavior from high end video software, I certainly will not accept it from a text editor.
Following that the high RAM use can lead to swapping which will impact performance and battery live. As I said above after a clean install, and with just one short markdown file open, Atom is now using over 2GB RAM and Climbing. Fortunately, I have 16GB RAM on this system, otherwise I would already be swapping.
BBEdit and SublimeText never tax the CPU unless I am working with extremely large files, or doing search & replace on a huge number of files. I often will have 50 to 100 files open for days in these editors.
I like the evolving ecosystem of Atom, it now looks to be about on par with Sublime, and I can see that it will soon over take Sublime on that front. But it really need to get the resource utilization under control.
To be fair to Atom, I also tried installing it on a Mac Mini with an almost clean Mac OS X 10.11 installation. I had similar problems.