So far, his ability seems to only go as far as to create anything that has already existed within the limits of synthetic and natural material, and *doesn't hold too many complex mechanisms*.
I think the issue with the smartphone is that the complexity factor included electricity. So while he could theoretically create all the physical components to a smartphone (the wires in every form of metal they come in, the plastic casing, etc...) it wouldn't work because of the electricity needed to run through it.
I think this implies that the reason his power, while kind of OP, isn't god-like is because he has even more limits on "natural materials"? Things like wooden swords are obviously no problem for him (given the recent chapter), but I guess the more intricate functions are impossible for him. He might be able to create something out of pure bone, but flesh and nerves and muscle + electrical synapses to get the body part actively working to any capacity doesn't seem possible for him.
I dont think forging a body part is possible, bcs that is a lot more complicated than a smartphone with all the nerves and all.
He once said that a generally simple mechanism device is doable, but complex things like smartphone isn't... something that use a chip or processor, perhaps? something that used very advanced technology?
But, then... how about 'forging' a missing/lost body part? then again, if it was possible, it will return to nothingness after Kaname's conscious control got released...