Weaknesses:
No matter how experienced or superior a team is, there always remains some flaws that other teams can exploit. Barcelona have had a good transfer window and have a great chance of securing the treble for the third time in their history. Nevertheless, there are still some imperfections in their team which could prevent them from doing so:
#2 Lack of competition for Suarez

This may sound a little less concerning, but believe me, this factor could again be the reason for failing to perform well in the UEFA Champions League. Luis Suarez is now 31-years of age and his outputs have started to drop, especially in the Champions League.
The Uruguayan has been virtually non-existent in the UCL for the past two seasons, having scored just 4 goals in 19 appearances.
A player with the qualities of Suarez must be scoring tons of goals in a competitive tournament like the CL, but instead, the team has relied on Messi for goals. The dip in Suarez's form is clearly because of both lack of competition as well as the lack of rest.
Neither Paco Alcacer nor Munir can match Suarez in terms of his finishing and guile, and this makes it tougher for Valverde to sub-off Suarez during games. The transfer team tried to bring in Antoine Griezmann from Atletico Madrid but the Frenchman chose to stay at the Metropolitano.
Timo Werner failed to please the Blaugrana hierarchy at the World Cup and that might be one of the reasons why Barca didn't go after him as he was heavily linked to the Catalan giants.
As a result, the problem still remains, and only better performances from the likes of Paco and Ousmane Dembele could help in overcoming this shortcoming. With a couple of weeks still left before the Spanish transfer window slams shut, there's still a possibility of a forward joining Camp Nou.