Green ending is space nirvana. I mean, the catalyst, Legion, and Javik are basically right; the differences between organic intelligence and artificial intelligence are monumental and in theory inevitable to clash. The technological singularity is a legitimate point of discussion today, and there are numerous interesting talks and papers to read on the implications of runaway intelligence that vastly exceeds our own biological limitations, including the unintended risks such a situation poses.
In that respect synthesis is literally the solution to this legitimate problems; the merging of organic and synthetic identity into a singular whole, where organics are no longer limited by biological limitations and thus ascend into a new being. There are legitimate theories that this is our very real fate, too.
But, you know, digging up the past, the issue is this theme of a technological singularity isn't explored anywhere near as thoroughly and deeply as the catalyst conversation seems to imply. It's not a facet of a vast majority of Mass Effect as a trilogy. Hell it's briefly explored in Mass Effect 2 with the Geth space station and then immediately discarded in Mass Effect 3. So by the the time it rolls around in the ending it's not a particularly convincing or interesting talking point.
And I mean, the ending presents synthesis as literally the best case scenario, especially in the vanilla release before all the DLC rolled around. It legitimately addresses the 'problem' outlined by the catalyst and the reapers, it's a legitimate solution to an actual problem that is thought experimented today, it has 100% preservation of life, cultures, species, and existence within the lore, ascends the entire galaxy beyond a technological singularity, and it's mechanically only available once a high state of content and mission success is explored and experienced. It requires the best game state.
But as said the real issue is it's inherently unconvincing, as are the other ends, because of the hot garbage writing the entire ending encompasses. The entire very last sequence is so dreadfully conceived in every way.