My understanding of Windows Hello is that it is just a 2D photographic scan, using IR for low light - but otherwise just a normal webcam. As such it is not as secure, and, in particular, can be fooled by a photo of the user's face.
FaceID, on the other hand, uses its dot projector, combined with the IR camera, to produce a 3D map of the face and is, therefore, more secure. It's this component that is more problematic at these sizes.
Certified Windows Hello uses 3D depth sensing as well. Which is why so few devices use the proper implementation of it.