In the past, I think people did indeed tend to sweep the question of where the impactor went under the carpet.
In the past 10-20 years, it’s become possible to do full hydrodynamical simulations, and that’s where things start getting awkward. You need a pretty big impactor to knock a Moon-sized chunk out of the Earth. And in the vast majority of orientations, most of the impactor ends up in the Moon, with relatively little in the Earth. That totally mucks up the Moon’s composition.
You can get it right, with precisely the right orientation. From memory, a low-velocity and fairly oblique impact. But, the skeptics would ask, what are the chances nature would manage to play exactly the right snooker shot?
In the new era of robotic exploration of the Moon, I suspect we’ll see a lot more debate about this. The Apollo missions returned lots of rock from a small number of sites, and some people question how representative they actually are.