I do think you need to consider the bootrom exploit already provides the tethered portion. With a tethered JB, you already have write access to everything and I would imagine that makes it much easier to configure the kernel and all the other stuff to make it untethered
However with the newer devices, there's no form of tethered JB so I don't think they can be compared directly with each other
However with the newer devices, there's no form of tethered JB so I don't think they can be compared directly with each other