Actually yes in hindsight you do look at almost everyone of those players and understand why they flopped
Di Maria - never wanted to be here, and was played anywhere but the position he excelled at in Madrid.
Sanchez - when he was bought all fans assumed he would play RW, alas the manager played him in the one position where we were already stacked which in turn had knock on effects for the two young players making that position their own
Schweinsteiger - legs gone, Bayern practically giving him away
Falcao - off the back of at least one major knee injury
Kagawa - to this day I dont understand why we bought a number 10 when he was never going to play ahead of Rooney or Welbeck. Fergie even said before we signed him we wouldn't be changing our style of play to accommodate him
Now you'll obviously argue that hindsight is a wonderful thing but you wouldn't of needed hindsight had proper scouting gone on before hand