I agree it is an upgrade for UM. The net result of it all is that UM has a better OC.
But in a situation where the AD calls the shots on who becomes a key assistant coach without the Head Coach leading and driving the process and making the actual offer/selection (not necessarily convinced that was what happened here), there is a chance for a dysfunctional relationship between the newly hired assistant coach, his Head Coach, and the AD.
Who does the new assistant coach feel he works for? Who is he accountable to? Who is he working to satisfy every day? Who feels they are on the hook for making sure his hiring works out?
The answer to all of those question can easily be be both the AD and the Head Coach. That can create dysfunction.
But I also think if that happens, it is fairly easily remedied. Early on, the AD sits down with the new assistant and the Head Coach (together or separately) and makes it clear that the Head Coach has all the responsibility, authority and accountability for the entire coaching staff and the team. End of conversation. Now go win 11 games. From there the AD communicates with the football team and staff via the Head Coach.