#2 He is not used to playing as second fiddle
In the French National team as well as for Atletico Madrid, Griezmann has been the main man and the talismanic player for every team he has played for. He is the one entrusted with the penalties, the corners as well as the freekicks and has enjoyed a free role behind the striker with no defensive responsibilites, both under Diego Simeone at Atletico Madrid as well as under Didider Deschamps for Les Blues. He is often the player that initiates attacks and dictates the tempo of the game.
Unfortunately, a move to Barcelona will see him come up against arguably the greatest player of all time in Lionel Messi, which means the Frenchman will be forced to play second fiddle. Since they play in the sames areas of the pitch, Griezmann's natural roaming playing style might hamper the Barcelona attacks rather than benefitting it.
He is likely to be restricted to a less free roaming role and if he is to play of the wing, will have to track back and defend like Ernesto Valverde is forcing Coutinho to do this season.