I get the "hood ornament" thing but I think there is a slight twist in the exact case of Braxton Miller this year.
The route running part, imo, will not be that difficult for him. I believe it's the blocking that might fall short and that is ok in limited doses. Unlike Wilson at the time, Miller won't be the only capable H back on the roster so Meyer can make a package for him that minimize that.
Bottom line is you (as a DC) still have to respect the threat of him getting the ball on the jet sweep and the decision to give it to him or fake it comes on the field, during the play from a QB that excels at making decisions like that. There is only so much you can scheme for there.
I'm sure the X and O experts can say it better but in my mind, Miller at H back will put an extraordinary amount of pressure on defenses in the run game if nothing else.
Obviously, the more complete an H back he is (running routes and blocking) the more of a threat he is but just the threat of his running alone is no small thing.