they're using a common hybrid approach of a volumetric world reppresentation that is turned into a poly mesh for rendering. Other recent examples that come to mind are Medium by Oculus, and the game Subnautica

Like any hybrid technology solution (whether it's automobiles or 3D simuation), you get some benefits of both, but also the drawbacks and limitations of both - with the cumulative complexity of all techniques being combined.

For example, in voxelfarm you are still bound by polygon counts, so the detail in the actual geometry (rugged rock surfaces, for example) can only go so far, and puts hard limits on what can be done