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 is a very tough one to handle because you are playing like the league format and so many teams will be faced and that alone can be very challenging. Combining the qualifications from the group stage with the games in their own leagues games is really a tough one and after this qualification it is true that the focus becomes even more intense on chances to win the title or go far in the league to at least have a decent finish.