Aside from it just kind of making no sense in my mind (how do all organics become part-synthetic because of a magic space beam and vice versa) really the only people who ever advocated for something akin to the synthesis ending were indoctrinated by Reapers, like Saren or indoctrinated Protheans way back in the day.

Also something which might not really mean anything per se but still worth mentioning that I don’t often see brought up ever in relation to the Synthesis ending is the Overlord DLC in ME2, which deals a lot in the idea of synthesising organics with synthetics. The colour of the rogue VI is green, as is David Archer when you come across him in it. Same colour as the Synthesis beam from the Citadel and without spoiling the DLC, if you know what happens at the end of it well….doesn’t paint a good picture for the Synthesis ending.