I think that the nature of the thing that was needing to be implanted would determine whether it was easier and safer to use a traditional incision. Hip replacements for instance use a titanium joint. However, seeing as the bone that it is replacing was not made of titanium, somehow nature has found a way to make bone strong enough to do the job. There is an important sense in which nature 'builds things using 3D printing' and does it without needing incisions, so even if we do manage to master doing 3D printing under the skin the way we do it now to an extent where we can replace things that currently need materials that are not suited to under skin 3D printing...