If you're evaluating VM hosts (proxmox, hyper-V, vmware, etc...) You need to have support for nested virtualization all the way down. Otherwise, if you want to evaluate a VM infrastructure, you need to start with bare-metal. Really, you just need to make sure that your top level support nested virtualization, but I understand their point.

However, the point about firecracker VMs in place of containers I think is really a good use-case. Firecracker can provide a better isolation environment, so it would be great to be able to run Firecracker VMs for workloads, which would require that the host (and the VM host above) support nested virtualization.