Paul Mitchell has only been DoF once, at Monaco. Before that he was Head of Recruitment at MK Dons, Southampton, Spurs and RB Leipzig. At the latter he worked under Rangnick, who was the DoF at both Leipzig and also Red Bull Salzburg.
Rangnick was the main man in building the Red Bull football system, being the DoF at both Leipzig as they went from the German 4th division up to one of the stronger teams in the Bundesliga, and Salzburg as they began their domination of the Austrian league. In saying that, both teams had a significant financial advantage over all their competitors so it could definitely be said that he only did on par with expectations. It would have been a massive failure not to achieve what they did.
I'm tempted to say that I would want both here in the same way they worked together at Leipzig. Rangnick the DoF and Mitchell the Head of Recruitment. Not sure if Mitchell would go for that though since it's a step down from the role he held at Monaco, although a debate could obviously be had where the Head of Recruitment at Man Utd is in the pecking order compared to DoF at Monaco. As long as things go well it'd also give him a big advantage in taking over as DoF in the not-too-distant future since Rangnick is getting on.