What do you mean he "failed" at Roma? He took a side that finished 6th and finished 7th while introducing a different playing style. Hardly a big failure, even more so considering it was his first job as manager in first division football in a completely alien and difficult environment for him that is Serie A.
With Celta he has taken a side that escaped relegation on the final day of the season last year and finished equal on points with Valencia this season while offloading the world class talent that is Aspas for € 9M to Liverpool and playing beautiful football with the ressources at his disposal.
His achievements with Barca B don't need mentioning.
He doesn't need to show that he can create a "system" or "identity", although he did with Celta this season. Barca already have a non-negotiable style and identity in place. The most important factor is that he knows Barca. From being a player to being a youth coach, he knows this club, its style and everything inside out. On top of that he is leader personality like Puyol. He always put in the effort as a player and he'll do so as a coach.
Your alternative would be?
He's coming to Barca with huge amount of pressure. He will be expected to hit the ground running from the get go. Early slip ups in the league and BCN press will be on him. They won't give him as much time, like they gave Pep. IMO.