Even though Real Madrid have a long history in the UCL and have won 15 trophies, it doesn't mean they are unbeatable. Real Madrid just lost points at Espanyol's headquarters in the continuation of La Liga, the negative result had a little impact on their mentality as they were preparing to face Man City who were much stronger than the team that had just beaten them.
This match is like a final for both teams, although Real Madrid are favorites, Man City can also provide a surprise.
True they are not like unbeatable, every team can be beaten even by the list team you expect but for the case of Mancity , I still don't think they will beat them and it's somehow obvious to me after what they played against PSG.
Not saying they can't though but if they eventually beat Madrid( considering squads and likes) then they stand to finish among the top 5 this season.. it will be hard to take out Liverpool anyway but they also stand a chance to win the title as well if they bear Madrid .