I can't find any reason to think the Barcelona team is still weak. Barcelona conceded just 20 goals in La Liga last season. Their defense is strong. They have shown that their defenders have the ability to perform well.
Lewandowski, the main player in Barcelona's attack, is quite talented and experienced. He was the top scorer in La Liga last season. In the friendly matches, we have seen the performance of Barcelona's young players Ansu fati and Raphinha improve. At the same time, Dembele is playing very well.
Barcelona have signed Xavi Gundogan to bolster their midfield. Barcelona's defense, midfield and attack are now strong enough. This is why I think the team has the potential to do well in next season's Champions League.
Actually we can't guess from a defensive point of view they are strong in the domestic league whereas we are in a big UCL game and in fact it's a different class, so it has to be from the UCL championship point of view how barcelona failed last season, that's why Xavi has time to fix his failure in Last season's UCL at least saw how its ability to overhaul players with a better strategy to win the league last season.
If you look at the preparation that has been mature right now maybe barcelona has looked better enough with the arrival of Gundogan also maybe they will improve in their midfield, I think this is the time when Xavi can show his quality as a coach and show other great clubs that exist in UCL, at least Barcelona has risen from adversity and maybe could have made it to the UCL final.

I watched Barcelona's performances in the Champions League and Europa League last season. I think Barcelona doesn't give a full effort in Champions League and Europa League matches. They gave a full effort in La Liga matches. And as a result, they won the title. Barcelona's performance in La Liga has been very consistent. But Barcelona's performance in the Champions League and Europa League matches seems to be very weak.
But winning the Champions League title is very difficult. Barcelona now have a strong squad. Coach Xavi is very hardworking. If Xavi can keep the Barcelona team stable next season too. And Barcelona's match strategy is better. So Barcelona has the potential to do well in the Champions League. But I think Xavi should make some changes in his match tactics to do well in the Champions League.