Now I will say, with certain models it fucks it up a little bit but normally it's fine. There's a thing for unity called Fbx exporter. So basically to make it work you need to make a prefab in the hierarchy but remove all physbones, colliders, particles, constraints, descriptor. Practically everything, except the mesh obviously. I think maybe you're supposed to keep the animator with the avatar rig thing in it. Sorry if this wasn't very clear.
