KJay, at the risk of beating a dead horse ....
Most fixtures have a little bracket that bridges the gap between the back of the fixture and the attachment to the box. This, IMO, allows for both solid support and a reliable ground even when the box is somewhat recessed.
I'm comfortable with that arrangement.
I'm not comfortable with any arrangement where the rear pan of the fixture is deformed as you tighten down the mounting screws, in effect acting like a lock washer over the gap.
That's what you have if my fixture is installed on a flat wall; you have a 3/8" (or is it 1/2") gap between the back of the fixture and the wall. You'd have the fixture secured in place only by the tension of that back pan; the screws will never be 'tight.'
The alternative is to have the pan tight to the box - which raises the question of how the wires will enter the box.
