It is currently difficult to move a decal on the surface of a union without reuploading it. This is why I like the behavior of SurfaceGuis with their Position & Size properties on ImageLabels however they do not snap to the surface like a decal.
Probably doesn’t happen because they’d have to basically convert your gui into a texture in real time and then put that over the mesh. If the gui is going to be static in your live game then you might as well just make it into a decal / texture.
It’s a very small case, but I would personally use this behavior to make x-ray vision on other players. Currently decals don’t offer light influence control (last time I checked), as well as not offering an ‘AlwaysOnTop’ property.
That is a very small case though and I don’t imagine it’s enough to call for this behavior on its own. However, my point is that decals cannot just be used in place of a SurfaceGui even if you’re using a static image.