Generally kernel level attacks and neighbor performance impacts on the security side.
On the functional side without a kernel per guest you can't allow kernel access for stuff like eBPF, networking, nested virtualization and lots of important features.
theoretically you can get to fairly complete security via containers + a gVisor setup but at the expense of a ton of syscall performance and disabling lots of features (which is a 100% valid approach for many usecases).
I think eBPF is a valid example, because it allows you to program the kernel to some extent. That being said and assuming it's not important to your individual goal, why is a rootless podman container running rootless podman inside the container still not sufficient? Do you really need nested virtualization? What are some of those other important features?
On the functional side without a kernel per guest you can't allow kernel access for stuff like eBPF, networking, nested virtualization and lots of important features.
Here is a good blog from docker explaining how even the best container is not as safe as a MicroVM https://www.docker.com/blog/containers-are-not-vms/
theoretically you can get to fairly complete security via containers + a gVisor setup but at the expense of a ton of syscall performance and disabling lots of features (which is a 100% valid approach for many usecases).