The group stage is going to go on very comfortably for Real Madrid. We must have all understood it from their winning against Napoli away recently. That was technically the toughest match of the group. But even Napoli couldn't stop them in Naples. I can already be confident that Real Madrid will finish as group leader.
Napoli lose to Real Madrid at Clement Turpin Stadium after winning Sporting Braga away from home in the group stage's first match, and they are the strong opponents that can stop Real Madrid from topping the group because Union Berlin and Sporting Braga did not come close to challenging Real Madrid because of their Champion's League experience.As it stands, Real Madrid will top the group if they keep consistency, because I know that they can beat Napoli at Santiago, and winning the remaining two home and away is not a problem for them.
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The important point is what they will do against the biggest title contenders. Especially against Manchester City and Bayern Munich I mean. If they manage to show a strong team performance with the help of their main scorer Bellingham too then I wouldn't be surprised to see Real Madrid adding another CL title to their collection. 
Hmm, this is just your imagination, and you centered it only on the teams you think can win the title, not considering teams like Arsenal that are in form and have the ability to defeat any team this season because of the talented young players in their squad. Nevertheless, it's your prediction, but remember that there are teams like Barcelona, Inter Milan, and others that are also capable of winning this league.If you mean that Union Berlin couldn't challenge Real Madrid in regards to the number of chances both teams produced, then you are correct. But it was still very close for Real Madrid to drop 2 points against Union Berlin. These smaller teams aren't without chances to at least keep a point in those games against Real Madrid and Napoli, but for Madrid the most important game was the one against Napoli in Italy. That game was like tossing a coin as Napoli could have won it as well. They were equally performing the whole game and Madrid had a bit more luck. It may have some consequences for Napoli if they make a mistake against one of the weaker teams.