Why are you being aggressive, dude? I didn't say that it was unreasonable to look at his record more closely and ask whether he's the best pick for united; that's a fair and interesting discussion point, and it wasn't trying to 'school' anyone. I'm just saying that his record clearly isn't solely composed of 'failures' you mention, otherwise he'd have a negligible professional reputation.
I'm not an expert on Mitchell's career but obviously his name has come up in various articles over the years, including when he was linked with a job at united or more recently for Chelsea, and they would mention various names (including Nathaniel at Southampton, who they sold to Liverpool; Toby A and Dele and Son at Tottenham; Tchouameni at Monaco, RB's american contingent) in the papers/athletic/aggregators. Obviously he wasn't the only person involved in those deals, but same would go for the guys you mention...