Isn't that exactly why they shouldn't come to Barca?
They play in a different team, they come to Barca it's a new team
They play in a different style, they come to Barca new style
They play in a different position, they come to Barca new position
They come to Barca new teammates, here they meet even more creative players like Messi, Neymar and so their role ends up changing completely,
They are no longer able to try and try different moves everytime because first they are not the most creative, they are the new guys, not as trusted
They will not be the ones given the ball by the team to do something,
they will be the ones to give the ball to Messi/Neymar (more creative)
So basically they end up playing nothing like what they used to be playing in the former teams,
Doing nothing like they used to do
And playing with MSN requires more defensive work for the new midfielder
So basically from the offensive player you fell in love with to a workhorse for MSN see Rakitic
Another example is Arda, as you said
Played in a different team, playing a different style of football, more direct
In his team he was among the most creative players, so he was given the authority to dribble, try make something happen etc
He shined and Barca bought him
Come to Barca, he finds that he is not as creative as MSN
He is made to play a new role as a CM
He is called upon to do more defensive work, because there are more creative players than him,
He fails in that because it is not really his specialty
At ATM he did a little defending since he had plenty of help from Gabi, Tiago, Koke, etc while at Barca there are so many players who are exempted from defence
He ends up failing
For me these are enough reasons to why Coutinho, Bernardo Silva, Eriksen, Dele Alli and similar players will be a failure at Barca