I agree that the outcome of a signed player is unpredictable, but I don't believe it's a gamble. The people in charge of scouting players should be professional on the job, and they need to work hand in hand with the coaches of the team to know the kind of profile they need for the team and the areas that need to be filled. That's why both the coach and the recruiting/scouting team should be very good at their jobs.
Now, even with all the right things, the player can still come in and flop, but with the right decisions, there's a lower chance of the player flopping.
With the case of Manchester United, I don't believe they make the right decisions when signing players. There's no single player that United have signed in recent times that has surprised anyone. They don't improve, and they don't fit into the team. All the players they've signed, they have turned out exactly how I predicted them to because you can see that there's nothing spectacular about the player despite the initial performance.