I do think their is debate as to if namespace containers are more or less secure the kvm and qemu VM's, I think the surface area of kvm and qemu is still very large and difficult to reason about. I think on some cpu architectures virtualization can be implemented on easy then x86 or x86_64, I think I read how risc-v have a much simpler and easy to work with virtualization instructions.

The surface area of the virtio driver should not be underestimated either I think.