Sorry, I think that's just clichéd rubbish, an excuse for any player not working out, "oh he just couldn't hack the mental pressure, he's too weak".
It creates an environment which is suffocating, and gives new players no time to get their feet under the table. Yeah, some players might be able to take months of abuse and criticism constantly and not let it get to them, but others might not, and that doesn't mean they are bad players, or aren't good enough, it just means we've created a scenario in which everything is stacked against them and ultimately have lost out on a good player because of it.
Bravo is another example. He has managed the mental pressures of playing for Barcelona, winning trebles, captaining his country to Copa America titles, but he arrives here under a complete media storm because he's the most expensive 33 year old ever and even worse, he's not Joe Hart, his first mistake is front page news for a week and he naturally takes a huge hit of confidence.
Bravo isn't a shit keeper, he was plenty good enough to play for Barca, but he arrived into a scenario where absolutely anything except instant perfection was going to be instant failure, and the more often we do that the more often we're going to see big money signings fail because sometimes they'll not hit the ground running for whatever reason.