Many good points have been made here. This is my take on the matter;
#1 D jumped in at the very beginning of this stuff, not only the beginning of my drive unit availability, but also of RC use on bikes by the general public as a whole. This makes for some probable issues right up front.
#2 His rear suspension allows lots of room for batteries, but it is tough for a non-hub drive, no matter what type.
#3 Etard makes a good point. These motors were not specifically designed for bike use, nor was the Kona designed for electric use. That makes for a more challenging project. Though most builds here fall in that catagory.
The primary issue with hub motors and off road use is unsprung weight. In order to get power in a hub, you need weight. This makes the suspension act funny. You get wheel hop and other undesireable characteristics. Plus, hubs move the weight to the outer extremity of the vehicle, when in reality, you want a centralized CG.
I would say if D was riding on fire roads and trails without any real jumps, a hub motor would be just fine. If you want to jump, however, you need something else or your handling will suffer.
If I were to build a FS mountainbike using RC drive, I would pick the bike specifically for the task. There are frames out there that are better than others for this type of thing. I think the Kona is a great frame and may even be fine for this project. But, D put so much time into the current build that it is getting frustrating trying to make it just right. I am blessed with my own machineshop. I do not have to rely on others nearly as much to make my builds successful. D is not that lucky. He must rely on a few trusted people to make his builds happen. This is not really bad. Lets face it, we all rely on others through our lives. But, it does complicate things when you are shooting for something that will truely be special.
What D is trying to accomplish is possible. It is just not common.
Matt