In the hope that this thread is not beyond redemption...
Look, when you are introduced to a sci-fi universe, you need accept certain deviations from reality in order to make the universe interesting (remember, science fiction). in Mass Effect,s case, this means, well, the the titular effect. Eezo can alter the mass of an object when going through an electric current. There's lots of technical sounding mumbo-jumbo behind it, but the gist of it is that this phlebotinum allows you to say ''screw Newton'' and go faster than light while lifting people with you mind and all that assorted jazz. It's accepted by the viewer because the entire universe is based on it; they didn't name the series after it for nothing. ME is hardly the only sci-fi franchise to use such things; in fact, as far as soft-hard scifi goes, it's probably closer to the middle, going a bit softer as the series progresses (Project Lazarus...). But mostly everything is explained in-universe, Biotics get several Codex pages, as does space travel, its implications and its limitations. It's not space magic because it doesn't just happen; there's a detailled explanation behind it and it may seem ridiculous at times, but again, science fiction.
Synthesis, however, just happens. All we know is that Shepard's ''energy'' will be ''dispersed'' in order to ''combine organics and synthetics into a new DNA''. And it will do this to the whole galaxy, trillions upon trillions of life-forms, instantly. That's all the explanation we ever get. I gave all the available explanation for synthesis in two lines. For a series that at least tried to be consistent and generally grounded itself into (soft) science, that's way out there. We can theorize how Destroy works (sort of an EMP pulse that disables synthetics, nothing out of place), we can understsnd how Control works (flip a switch, Shepard is the new manager, orders Reapers to begin their galactic community service at once) but Synthesis is a feat of technology that seems far beyond the rest. Affecting all the Reapers (easy, there's what a couple thousand of them? Cerberus themselves were making progress on that front) or all the synthetic life-forms in the galaxy (there aren't too many, and they all have Reaper code for easier tracking, hell maybe that's why EDI and the Geth absolutely have to be destroyed) is one thing, but every living being in the galaxy? Changing them to the very molecule without any sort of known side effect and making them perfect peace-lovers who glow green instantly? That's a whole another scale. It breaks the rules of the universe by virtue of just being too much, not being explained, and not seeming to belong into the technical limitations and established themes of the universe. Being able to bend mass with a specific fictional element does not mean that a radical change down to the very atom of each and every being in the galaxy instantly and harmlessly is acceptable, period.
An analogy; take Fallout. It has magical radiation that does all sort of things, from making scorpions huge and aggressive to turning humans into very long lived, cancer-ridden Ghouls. It has portable laser and plasma weapons (which is very high tech even in the ME universe). It has all sorts of implausible science-y stuff, culminating with a person being separated from their brain and still living, to the point of being able to hold a conversation with it. Why? 50's Science! meets Rule of Fun/Funny. It's how the universe has worked since day 1, and it's accepted as being part of the setting. That does not mean that anything goes, however,; if Gandalf suddendly appeared and cast a spell that make the Earth radiant and beautiful again and brainwashed every Raider out there to hug fluffy bunnies and offer cookies to passing folks, fans would call bull****. Why? It's not less implausible than having a chat with you very own disembodied brain. But it comes out of nowhere, is not even explained one tiny bit, and it generally ruins the setting.
That's why it's viewed as space magic. Not only because it's implausible/impossible, but because it doesn't fit the setting at all. If this was 2001: A Space Odyssey (which was only hard sci-fi when it wanted to, the rest was pure fantasy) or The Hitchhiker's Guide to the Galaxy or another piece of non-serious or utterly esoteric sci-fi, Synthesis would pass as normal. In a setting that at least tries to take itself seriously, it just utterly wrecks the willing suspension of disbelief.
TLDR: It's not only that it's implausible, it's that it doesn't fit the universe and comes completely out of left field to radically alter the plot and the very fabric of the setting.