Griezmann playing for Atletico and for Barca surely should be two completely different things due to playing style.
Therefore his previous positions shouldn't scare people - only if he wasn't an intelligent player, which he obviously is.
And just a reminder: Last season Real Madrid won the league and CL not by playing well, being innovative in tactics, but simply by having the best squad in the world. Great team-members all around can also win you trophies, and adding Griezmann to this squad can never be a bad thing. Tbh, he is future ballon d'or potential with his national team, and with improvement here at barca (when messi is close to retirement).