Barcelona's performance under Hansi Flick is indeed extraordinary, the coach did not need much time in his first season, he immediately managed the team very well as he has shown now, and indeed there is no need to doubt that Barcelona is a strong candidate team to win the UCL title, they only need to maintain their level of consistency in this increasingly tight schedule.
As a Madrid fan, I doubt Barcelona can win the UCL title this season. 
I admit that Barcelona performance is better now after replacing the new coach and in the end Hansi Flick proved that he is very suitable to be Barcelona coach even though he was initially doubted. But no matter how strong Barcelona is now, I am sure it will not be easy to get through all these challenges because there is still Liverpool who is much more consistent and has better quality players.
"I can't say that Barcelona will be the team to win the Champions League this season, but, to tell the truth, based on their performance so far this season, Barcelona is among the top three teams favored to win. If we consider the best teams likely to win the Champions League, Barcelona’s performance hasn’t been bad at all.
As for challenges, I don't think there’s anything Barcelona can’t handle recently in this season Champions League competition doesn’t seem as much competitive, as many of the big teams are not in serious form. So, I don't think there will be as much challenges for big team in serious form as in the last three seasons, when top teams were great form.
I can’t say much about which team is likely to win the Champions League this season currently because the tournament hasn't progressed far enough yet. If it were already at the quarter-final stage, it would make more sense to predict, as any team could still pick up good form by then.