The fundamental complexity of voxel data structures is higher: O(n^3) for iteration, and the density of polygons required after meshing is generally much higher. The only real benefit from a complexity standpoint is that storage and access of specific voxels is much faster, but this often doesn't represent a payoff.
Voxels are great for a lot of things, but making a game like Veloren run quickly is much harder than you might initially realise. There's a reason that we're told at least every 3 years that a new company has come up with the 'engine of the future' using voxel tech only to have it vanish into obscurity as they try to turn it into something that's not a tech demo.
For Veloren, there is a big benefit: we get to create enormous procedural worlds. But actually controlling the complexity of that data and efficiently storing/accessing it is a non-trivial change as you'll see from the project's dev blog.