On that basis no-one would sign anybody. Of course no player is guaranteed a success, but I think the better the player the more chance there is. If Messi signed for Burnley he'd still be amazing, maybe just not as effective as he is at Barca because of their tactics and the players around him.
Similarly, with Pep and our squad, most very good players are probably likely to improve and enhance the team. Nothing is guaranteed but a player like Sancho or Koulibaly is probably a better bet than say, Soyuncu or Grealish.