Advancing from the group stage is the big challenge in UCL because of the new format. Any club that is able to qualify from the group stage will become more focused with their matches because they have the advantage to qualify to the next stage if they win their opponent. It's from that stage that one can start to predict which of the clubs will be the favorite for the Champions League title this season.
The group stage in this new champions league format is a tough one seeing that you have to face a good number of teams combined with your domestic league games is a challenge in itself, so I will agree that getting past this stage alone is a big challenge and also an epitome of hope to make it to the next round of the league depending on the strength and performance of the team.