And FreeBSD Jails are still superior in the way they're a "first class citizen" in the kernel, vs. the "hacky" feel that Linux containers has.
I have no doubt that Linux containers are just as secure as FreeBSD Jails, but if the implementation and tooling is complex, there is a much higher risk of something being configured wrong.
And then there's the giant gorilla in the room, Docker, which probably has the best tooling of them all, and initially used Linux containers, but has since moved on to their own container implementation (runC, https://www.docker.com/blog/runc/).
I have no doubt that Linux containers are just as secure as FreeBSD Jails, but if the implementation and tooling is complex, there is a much higher risk of something being configured wrong.
And then there's the giant gorilla in the room, Docker, which probably has the best tooling of them all, and initially used Linux containers, but has since moved on to their own container implementation (runC, https://www.docker.com/blog/runc/).