IIRC from a presentation the main point behind NMBL is to not reimplement an entire OS in the bootloader like GRUB. Instead you should use the kernel with an Initrd instead and should kexec if you wanna boot into a different kernel. That way you only really need to take care of the existing kernel and userspace security.

The problem with that is that it starts to muddy the TPM PCRs (read: makes the PCRs that should be predictable not predictable) if the kernel gets kexec'd and it just makes the boot processes just needlessly more complicated. Not to mention when the kernel/initrd fails to boot you are kinda SOL since you can't really do any meaningful boot count logic if it fails as it could even be a faulty kernel and not even reach the initrd.

I also haven't been able to be convinced that NMBL is better than a simple EFI bootloader that chainloads a kernel.