Well in the Dragonball universe there are strong humans, but the problem with humans is that in Dragonball, power is tied directly to Ki.  Mr. Satan is strong for a human but as he has never figured out how to use his own Ki to boost his own power and abilities.  So few pure humans have figured out to use and control their ki and the humans that can never seem to go to the same lengths of training as other characters.

In general it does seem that humans have a harder time increasing their abilities naturally and increase in power at a much slower rate then other races but I don't think we've ever seen a human at 'full' potential, if there even is such a thing in Dragonball.  So in order to keep up with other races, humans do tend to have to go to extremes to boost their own powers through genetic manipulation, modifying their own bodies to become androids or cyborgs, using mystical items to boost their training and base energy, etc.  Another possibility that might be less intrusive on the body then the android or cyborg route would be injecting nanomachines into the body to boost ki production and enhance the bones and muscles.

I think the dragonballs should not be used as a way to 'unlock' or 'boost' humans but rather they should find their own way to increase their powers.

Well as you can probably tell, I'm keeping an eye on this.