To give you my impression about Rangnick over many years
I'd say he's a very intelligent, academic football teacher and a great strategic analyst. In Germany he is also known as "The Professor"...
He will change utd's way to play, their intensity on the pitch and their complete scouting and academy system. That's what he stands for - change. And I guess utd fans are exactly waiting for that.
This might be bad news as it seems the club owners have identified the necessity to change a lot. And he needs a lot of power and competences to play that role, which he likes. If utd want a puppet they won't be happy with him too long. He is quite demanding.
He might have some success as consultant or DOF after end of this season.
At the same time his strengths as a theorist and tactical expert have shown fundamental problems addressing players in the right tone, especially stars and divas running around the swamp.
I doubt Rangnick will have a strong impact as a coach to save the current season. But his mid-term impact could bring some trouble for non-utd fans. He definitely will change that wonderfully pisspoor transfer philosophy of overrated and overpaid big names. Rangnick is a man of youth talent development.
If he gets the power he needs, it could be some turnaround after years of misery. But he could also get on everybody's nerves in that club with his know-it-all manner - as a German. Thin ice for him...