Michigan is kinda like Minnesota, but on a bigger scale. Once it snows, they PACK IT DOWN, and you don't really get the "off piste experience" you might elsewhere. Coverage is usually pretty good because the first layer is usually man-made, and it covers a very large % of the resort.
That said, you're right about the groomed trails. A thick base of manmade that's squashed down on a regular basis makes for little chance of base damage if you stay on the groomed. The fact that it's grass instead of rocks underneath adds a layer of insurance for when you do find a thin spot.