Ranked in these QuestionsQuestion Ranking
Pro Automatic updates
Automatic updates means better security and reliability.
Pro Docker pre-installed
Docker comes out of the box.
Pro Built on systemd
Uses unit files for managing all services and journald for logging.
Pro Great documentation
Has guides for installing systems like Kubernetes, setting up etcd, and using cloud-config to manage hosts.
Con No package manager
CoreOS has only the minimum for being a host. It doesn't even have a package manager. Alternatively you can start a container with any software you need.