In my AC17 version, an SEO of "subtract w downward extrusion" does not work on CADimage Objective objects. This operation only subtracts, without any extrusion cutout. [EDIT: Scroll down below, for illustrative screenshots.]
Oddly (and as a workaround), if you add at least one non-Objective element to your Objective targets, before executing the operation, then the extrusion will happen. You can then delete the "dummy" non-Objective target, and the extrusions will remain on the other Objective targets (Another oddity is that this appears to be only true if the dummy element is actually affected by the SEO operation. If it is "off to one side" and not being cut, then when it's deleted, the downward extrusion on all the other objects disappears!)
I've found the easiest way to do this is to copy-displace one of my Objective targets (displaced only enough to be able to select it distinctly) , right-click > convert to morph > and then include that in the targets before the operation execution. And then delete it.
Is this bug still around in AC18 ?