So are you actually using PolyVox? If you have problems with MagicVoxel/VoxelShop then you should probably ask the authors of those packages directly. However:
iUltimateLP wrote:
What kind of tri / vert count is alright for a voxel terrain?
It varies a lot of course, but is generally quite high unless face-merging is applied.
iUltimateLP wrote:
...if I export it straight out of MagicaVoxel with the .obj format and import it into Unreal, I have no artifacts, but the tri/vert count is really high (~14K/~21K). Is that normal?
Probably, yes. Estimate how many 'surface' voxels you have, and you'll probably need several tris/verts for each if no face merging is done.
iUltimateLP wrote:
If I try lowering those counts by importing the .vox into VoxelShop and export it with the LowPoly exporter, I get those strange artifacts...
I've never used voxel shop so unfortunately I don't know much about how the LowPoly exporter works. Definitely ask the author about this.
iUltimateLP wrote:
I saw some twitter posts by VolumesOfFun (
https://pbs.twimg.com/media/CNSdVlXUkAAlch4.png:large) where I can't see those artifacts, so could anyone tell me facts about those two questions? Thank you

For out games we do not export objects as meshes and then load them into Unreal. Instead we use our PolyVox/Cubiquity libraries to load the voxel data directly, and convert it to meshes once in the engine. We do indeed to face merging to reduce the tri/vert count.
P.S. I also saw your tweet, hopefully the above answers it.