I always order cards for my AUs, although I suspect it isn't necessary.
I did have one Discover AU where the card never arrived at my address about two months ago. But everything else went smoothly with that AU, so I ascribed it to the USPS losing it in the mail, which has happened a few times over the past six months or so with mail in general in my experience.
As a general trend, and this is not Discover-specific, I have noticed some CC companies adding steps to complete *after* I enter all the AU information on the website. Things like please enter the three digit code on the back of the card, or please press the "Yes, I *really* want to add this AU" button. Seems to be more a general security feature, but it's easy to see the "AU added" message, assume everything is cool, and close out the window before doing the final final step. So now my practice is to watch carefully to avoid this mistake.