What I find interesting is these two projects are using a similar approach but targeting opposite ends of the spectrum. Atomic is intended as a cutting edge server OS for enterprises that want an integrated stack for running containerized infrastructure (they also started publicizing Atomic Workstation as of the last release, although I found it painful to use due to a lack of docs). Endless is targeting emerging markets, composed primarily of first-time computer users. OSTree+Flatpak allows the desktop to run similarly to a mobile OS, and there’s minimal chance of users breaking the system itself.
[1] https://www.projectatomic.io [2] https://ostree.readthedocs.io [3] https://flatpak.org [4] https://endlessos.com/for-developers