How much of a Mass Effect-style future seems plausible?

The biggest stretch is faster-than-light travel. We do not know of any real substance or technology that could make a ship travel faster than light, and the fictional drive material does not have a known scientific counterpart. Without some breakthrough that changes what we understand about physics, the galaxy would be much harder to reach and interact with.

That does not make every part equally far-fetched. Alien life is an open question, but a crowded galaxy of technological civilizations, all close enough for routine contact, adds several big assumptions on top of one another.

The social side feels more believable than the travel technology. If different intelligent species did meet, disagreements over resources, security, status, and who gets a say would make sense. Prejudice and crime are also familiar human problems that fiction can use to explore those conflicts.

What is less predictable is the shape of the institutions. A council representing very different species might not work anything like a human government, and we cannot really know what alien priorities or cultures would be like.

I would call it a useful thought experiment, not a likely forecast. The setting takes one enormous speculative leap with convenient interstellar travel, then asks more grounded questions about diplomacy and social friction. Those questions can still be interesting even if the technology has no established basis.

So, plausible themes, very uncertain alien life, and highly speculative travel. The whole package depends on the part we currently have the least reason to assume: easy contact across the stars.