We're happy to talk about what you need on the security front. Some of the folks working on Flatcar Container Linux have a very strong security background; worked on AWS' EC2 security team, do regular pentesting for distributed systems[1], have reported dozens of security issues to upstream projects packaged in Flatcar Container Linux, including the kernel.
We've just now started breaking away from the upstream project, and updating packages. Addressing any open security issues is front and center in our efforts. If you have concerns we'd love to hear them.
We worked with CoreOS team for years (was our founding project) and they trusted us on the security front. We feel that if you trusted CoreOS and know our team + background, you should have just as much trust in Kinvolk.