Real Madrid, Manchester City are one example. With the strength they have from the composition of players in the squad, they are believed to be a team that has a high percentage of chances of winning the title. Moreover, a team like Real Madrid is the one who wins the most, and every season they are the favorites to win the championship.
Even the teams that are considered dark horses here are also good teams in their domestic leagues, so all teams have to be wary of every opponent they face.
In the Champions League, it is hard to predict what will happen in general. First of all, there is a reason why those teams are playing in the Champions League. They are the best team in. That's why they are able to play in the Champions League competition in the first place. So of course every team is going to have to be careful while playing in the Champions League competition. Even Real Madrid and Manchester City.
Another thing is that the Champions League competition is probably the world‘s biggest club competition ever in any sport. so it is a prestigious title to earn. That's why every team gives a special commitment to win the Champions League competition. That is also a reason why we sometimes see accidents happen, or should I say surprises ?
This is why you see that only big and strong clubs at that moment are winning it. Real Madrid happens to understand this and that is why every season they make sure that they are strong enough to win it by signing new players always to improve the team performance.