The Moral Landscape is hard to disagree with -- because it's mostly right -- but I found a way! This will make more sense if you know the case it lays out.
Realism & consequentialism are awesome frameworks, and I share them. I wish they were more useful in resolving world controversies. Fatal problem: Well-being is terrifically thorny to even "in principle" aggregate or compare.
Let's say I hand you full lifetime printouts of Huck Finn's & Paul Atriedes' vitals -- that alien satellites printed everything about their inner lives in minutest detail (far richer details than the novels showed you): Are your common-sense judgments likely to change? How might you go about making a solid case that Huck's life was better than Paul's?
Make it more real: Now we have a thousand free-range kids (Hucks) & a hundred carefully-raised princes (Pauls). How do we compare them? (This really matters for kids!)
In natural experiments, so many confounding factors get in our way: There will never be a well-controlled longitudinal study of Hucks versus Pauls. Other huge factors will stay hugely correlated with factors we'd hope to isolate. (If we could ever see many, many more samples than plausible causative factors, we'd uncover surprises: but that might take more people than atoms in the universe.)
Our world already gives us as much useful data (if we could go around and look) as we're ever likely to get.
Many talented individuals make astonishing social perceptions. It's one natural human inclination, honed by evolution and sometimes by culture. We can do things to increase the talent pool -- and then listen to the best of them -- and that's the foreseeable limit of our useful perceptions.
You & I agree on many extreme examples: That some places are much friendlier to human flourishing than are others. Some places are so bad off that it's likely literally no one would agree to be born there. But, this is not terribly controversial (especially when we notice the moral relativists' agreement with us in their actions, if not their words!), and there's no real path -- even in principle -- from agreeing on the clearest cases to agreeing on harder ones. More data won't help: too many variables.
Today's careful human social perceptions won’t be beaten by machine assistance in the next hundred years. Our disagreements won't be solved by super-fMRI, but maybe by careful honest perceptions & sound judgment & trust. These are hard to build, but there's no other path.
Your "in principle" path -- hoping to help social policy reach the clearer agreements of physics -- doesn't actually exist. It would be awesome if it did, and awesome if people cared if it did -- but, for our whole foreseeable future, we'll keep relying on tools we have today.
Our real argument with social policy opponents will never be about the significance of brain scans -- but about the honest perceptiveness of dueling culture experts.