I haven't read these studies, but there are a few points that I'd bring up. It may very well be that ground hardness does not modify force on joints. However, research like this is frequently done in short bouts. If a runner was to complete runs on 6 different terrains, they would likely spend less than 5 minutes collecting data for each terrain. The problem with that is that fatigue is the negative aspect of barefoot running. If the terrains were measured under these conditions, they would not be applicable to real life bare foot running. And just to address the Indians: many humans throughout history have completed amazing feats even though their anatomy limited them. The Jews ran hundreds of miles starved, barefoot, and freezing. Does that mean we should train like that? No, not at all. Would I say that the Tarahumara Indians had a high incidence of stress fractures and bone spurs? Oh, you better believe it. We're trying to find a balance for optimal proprioception, form, and efficiency. I don't feel like running barefoot on very hard surfaces allows for that.
As you fatigue, you cannot resist the forces resulting from your foot strike. Because of this, more force is transferred to bones in the foot. If this happens repeatedly, as would occur in someone doing frequent long distance in VFFs, then the result is a stress fracture.
On top of that, you mention that the research says there is no extra force on joints. Firstly, what joints were measured? If it was the knee, talocrural, and hip joint, then that is to be expected. However, rate of force transduction through the foot (through all the small tarsal-metatarsal, metatarsal-phalanx, and other bony articulations) is much greater. Overall, force is the same, but rate of force transfer is likely to be different (which can significantly affect bone structure). Secondly, do you know the duration each terrain was measured?
I've actually been thinking about this a lot lately. My biomechanics professor (Dr. Keith Williams) has recently started talking about barefoot running as well. He has many years of research in the field of biomechanics, specifically with reference to running, foot strike, and form. He has been bringing up some points that I've thought about personally. I'll see if he has some research articles I can bring up.
Here is a great article that references numerous studies on running injuries: Running injury exercises - myths about running injuries. Unfortunately, I don't have access to the full text of many of those studies, so I don't know the duration of the studies or how the results would change over a 1 hour run versus a 5 minute run. Nevertheless, numerous studies show that people adapt their running and leg stiffness to the hardness of the surface, so I'd be surprised if the difference between a mixed terrain of dirt/rocks/grass/etc (as our ancestors probably ran on) would be different enough from concrete/asphalt to make a difference in terms of injury rates.
Moreover, the Harvard study of barefoot vs. shod runners showed that impact forces transmitted through the foot, ankle, and leg immediately after impact with the ground are about three times greater in shod runners using heel strake compared with barefoot runners using forefoot strike. Therefore, whether or not surface hardness makes a difference, it seems that shod running - at least using heel strike method (as the vast majority of shod runners do) - would be worse than barefoot. In other words, I've seen very little evidence to show that running shoes ever increase safety.
