[HS]

Hollow bones in robots seem like a good idea. First less material is required for equal strength. Second, you've got somewhere to put batteries or compressed air, or even liquid gasses for simultaneous cooling of electronics, heating of gas, and subsequent activation of pneumatic muscles. I really like the efficiency of the last idea, it all fits. By the way this is a foot seen from the side.

[Korrelan]

From an engineers perspective providing a robot’s structural rigidly through ‘hollow bones’ does seem like an intuitively good idea, but it does have many drawbacks. 

Besides the general access problems, the joints usually pose a problem.  Actuator linkages/ internal wiring etc have no easy routing solutions, designing efficient hollow joints as conduits is a major problem.  If you use standard internal joints you negate the gains of the hollow exoskeleton.

This is why the human model is a go to solution.  A rigid skeleton with inline/ internal joints to support the heavy internal components, then the actuators and lastly the covering/ armour/ skin… evolution wins again.