Barcelona walk into Vallecas as the overwhelming favorite, and everyone in the building already knows the expected script: the giant shows up, the giant wins, the giant moves on to the next storyline. Rayo Vallecano exist to complicate that script whenever they can. They do not have Barça's talent or their financial gravity, but Vallecas has never cared much about reputation. It is one of the most hostile away trips in La Liga precisely because the underdog refuses to play its assigned role.
This is the tension in every heavy-favorite trip: the bigger the gap on paper, the more it stings when the smaller side closes it, even for a night. Barcelona have too much quality to be scared off, but La Liga has punished plenty of favorites who treated a trip to a scrappy home crowd as a formality.
So the question isn't whether Barcelona are better. It's whether better survives contact with Vallecas. Do you trust the favorite to handle business, or is this the night the moneyline gets tested?