For me there´s always been a limit to how vast some effects can be and how good and realistic they can be, not a mish mash of stylized environments and minor effects.
So it would be vast full 3D environments that covers infinite distances, both in terrain and volumetrics, as well as there is no way to produce infinite cloud layers except employing ogo taiki, ozone or mix with vue and terragen.
Otherwise you need to do workarounds and approach a seemingly vast landscape with sections, and render passes, as opposed to apporach it with more physical correct infintie cloud layers and terrain etc.
Apart from that there is the multiple scattering thing, but can be worked around by the help of octane, but octane do not solve the issues above.
So I would say it´s pretty much that which is lacking, you can probably produce any kind of model and environment apart from that when it comes to creation.
another thing when it comes to advanced dynamics and interactions or crowd behaviour, where it can only go so far where others can take it to another level.
The tools today can cover so much ..but it´s probably more a question of How to approach things with the most efficient workflow and how fast you can simulate or render.
here´s a sample of scenery that would take you forever to try and replicate within Lightwave alone, and keep in mind that even this with Terragen would require a render farm most surely, but to get this level of realism in the sky and terrain..boy you would have to work your..ar..off and still not getting that level of realism in clouds.
The vfx breakdown also is also accompanied with some tutorials here, and in fact it is also using Lightwave in combination with Terragen.
so a typical example of what you "can´t" create strictly within Lightwave, not at that level.
some users have made some commercials, like for some aeroplane company ..which I think our fellow well known user Lino Grandi did som cloud work on, but it´s not on the same realism and same level, though I think they researched terragen as well and still opted for Lightwave, but if you have a good eye for how it should look, one can see where that commercial is not seamless in it´s cloud coverage..nor has the same realism compared to this Terragen sample.
I´ll bet they cranked up the shots faster though.
Another part is terrain sculpting, you can´t sculpt parts like you can in zbrush or blender, you are restricted to displacements and cant adjust them on the fly with sculpting where needed, thus terrain rocks will only have a specific style and can´t be tweaked Exactly as you want them.
And there´s more, but I think we can discuss around that for now.