It mostly isn't. Almost all Linux container escapes only require the ability to make system calls to the shared kernel from processes inside the container. The system container doesn't really restrict this ability. It also increases surface area to compromise the container before attacking the host system, since there's now a bunch of extra software running inside the container.

If privilege isolation is a priority but you want to use containers, gVisor and Firecracker are way ahead of anything else. The Linux kernel API has proved to be very hard to secure, and not for lack of trying.