Sheps organs seem to be partly synthetic. Miranda: "In an effort to accelerate the process [of recovery], we´ve moved from simple organic reconstruction of the subject to bio-synthetic fusion." But I don´t know what this exactly means.
Biological parts made synthetically, or made to heal through synthetic assist, making those parts synthetic, by some standards, but generally not. If you needed a working heart, you could go for a more artificial-robotic working one, sure, but a bio-synthetic one would work just as an organic one, with the same or nearly same elements, but still be synthetic. Also would reduce the chance of rejection.
Shepard is a 'hybrid' that is 'human'. Artificial means were used to reconstruct him, but he 'is' a natural being. And still 'just a man', albeit one that verges on being a transhuman.
A 'synthesis' was used in his healing and reconstruction, but the 'synthesis' isn't used for what the end product is, in ME2-ME3. You could have nanites used to do all sorts of things to you, and they could even lay completely dormant in you, but if you're otherwise 99.99% of the time operating as if you're a human with human limitations, then you're human.
I just want to say though, that this whole 'Shepard is a human' thing, which I do believe, is nonetheless not the whole truth. He is only human so far as he and others consider him to be a human, and by many of our standards, he is effectively transhuman and partly on the way to being a synthetic entity outright.
If you go mega Renegade, the transhuman shows on his face (if not hidden away by cosmetic surgery) but he personally denies it.
If you go mega Paragon, the transhuman doesn't show on him but he personally (relatively) accepts it. Or at least the concept of it.
Mixes of Paragon and Renegade could make Shepards that utterly reject anything robotic and just consider their implants to be tools, or Shepards that increasingly welcome synthetics and show just how robotic they've become on the face.
In any case, the narrative of the trilogy clearly does not want people thinking too hard about Shepard being synthetic, even if it partially acknowledges it.
We may consider people in the coming years/decades who undertake gene therapy and replace some body parts with cybernetics to be transhumans, but honestly, honestly... at that point (years/decades from now), I'm not sure those people would actually consider themselves transhumans, nor would the future society of decades from now. If it becomes the norm definition of human (as we still consider those who have hearing aids or heart implants or glasses or whatever), then it is 'human'. If it is on the transition away from 'human', then it is 'transhuman'.
Language is funny like that.
IMO the Catalyst is right when it directly states "even you are partially synthetic", even while Shepard can still be called 'a man' or 'human'.
But the lines are meant to be increasingly blurred. The Collectors have organic parts and come from an organic species - are they Prothian? The Geth may have an upgrade that changes their code to operate in a highly organic manner akin to human/quarian neural networks - are they still robots?
'Transition' and 'Post' become words necessary in order to understand states of existence. Collectors may be replaced entirely by tech, but much of that is an organically based technology. Geth may have been arguably called robots, but upon upgrade it is more difficult to fully consider them as such, but instead 'synthetic life' or 'conscious entities' at minimum, not robots.
Dragon Age is doing a similar thing, but instead with mortals and spirits.