Every signing a big club makes could be good in the right situation, but making a team, the right kind of players, a coach to work well with them and calculating their durability/injury risk is something that requires an eye. Otherwise I load up FM and say yes this Kvara guy is good and sign him, but put him in the wrong team and he's another dud signing you beat the ds with.
Often I think teams just fluke it a little bit, but there are lots of gambles like that. The thing is PL teams can afford to try again every year as they have the money. Some teams pick a potential great coach who turns out to be one, and they make almost every player look better. Others turn out to be great coaches at a certain level only, or they are great at one particular thing. It depends what stage of the team build you're at also.
Go to our future coach thread and you'll have all different opinions because no one has an actual answer, other than saying anyone will be better than Allegri. Which they might well do, as he seems to have lost some players. But there is no guarantee a proven "great" coach like a Zidane would transpose what he did at Real Madrid here, it's two teams in very different stages of growth. We need someone to pick it up and improve every area with enthusiasm.