The visuals of the prefabs are completely broken, and there are extraneous, unrelated parts; there is a high probability that they are incompatible with Unity.
Hello, after checking the FBX models the issue could be the included collision meshes are being mistaken for part of the model by Unity, if the assets are all weird blocky shapes this would be the issue. You should be able to work around this by importing the models into Blender as it will automatically separate the collision from the model, you can then export the models at which point they should work in Unity.
I'll have an update out within a few days to fix this issue as I was unaware of it.