I know it goes against "the message" but I would think if there was a significant undercount, it would be the more well off [[and probably white) folks who use addresses outside the city to avoid income tax and auto insurance.

Of course, the census has nothing to do with either of these and you could be counted on Detroit's numbers and tell the insurers and the city you live out of town, but many doing this probably wouldn't be inclined to take the risk anyways.