QUOTE: "Now we know why you do not see many engraved aluminum receivers."
It may be more difficult to engrave aluminum and be aesthetically pleasing but it can be done. A case in point would be the standard factory engraving done to all Browning Double-Auto shotguns, most of which had alloy receivers.
Aluminum is easy to engrave, BUT if you get too fine a detail too deep, it is not at all durable. I agree the work on this gun does not look professional, but perhaps they suited it to the gun.