I also ran into an error where a mesh wouldn't import at all and the error read
Could not find any valid mesh on the root hierarchy. If you have mesh in the sub hierarchy, please enable option of [Import Meshes In Bone Hierarchy] when import.
I have that option enabled so I figured it was something going wrong in houdini. All of my other meshes (Legs, arms, torsos) work as intended. Just the heads that are having issues.