Back when I had Jiongu, I kept it nice and simple; I had no bones, so Kaguya was out of the question. But I also had no muscles, which meant no Gates.
I also assumed that being Jiongu meant just that, no bones or muscles, but you'd still have everything else.
On the Earth Grudge Fear wikia page, it states about how Kakuzu used the technique to replace any part of his body, especially his organs. This is why Hidan and Kakuzu were considered immortal.
So, I like to think that with Jiongu you still have your organs but can easily replace them if need be, hell, maybe even skin.
And as for the whole not having a brain thing... I don't think we really need to go into details that far, right? It's obvious that Kakuzu, at the least, has a brain because he shows that he can feel pain.
For using Sasori as a reference, Sasori couldn't feel pain. His heart was pretty much his whole chakra system, hence why he was able hope from one to the next. But he never showed any emotion, or any type of understanding for pain. So, by that logic, Kakuzu has to have a brain.