Completely ran my MBP with 16 GB of RAM out of memory within half an hour on El Capitan 10.11.6 (had gobble 8 GB to itself alone). This has only happened to me once or twice before in five years. Lost fifteen minutes quitting apps and finding out how to run kill -CONT -1 to get the show back on the road. This is not a productivity app but an anti-productivity app. I'd be wary of Undistracted and the window managers of this developer as well. If Deskcover has these kinds of memory issues, his other productivity applications are also a risky bet.
I mention the other apps as I was considering buying Undistracted now as I like its set of functionality.