Real Madrid are sitting at the top of La Liga with 30 points from 11 games, 5 points clear of second place Barcelona, and they are the most consistent team in the league right now. Kylian Mbappé is in absolutely lebthal form with 13 goals in just 11 games this season, making him one of the most dangerous strikers in Europe, and he is perfectly supported by Vinícius Júnior who has 5 goals and 4 assists, giving Madrid multiple world-class finishers who can score at any time. Rayo Vallecano are 10th in the league and completely outmatched - they lost to Villarreal last weekend and they are weak defensively. Vallecano have won only 1 of their 4 home games this season, which proves they cannot handle top-quality opposition. The reason under 5.5 goals is safe is because Rayo's attacking problem is as bad as their defense - they scored so little this season that 4 of their 4 home matches stayed under 2.5 total goals, meaning even when Madrid score 3-4 goals, Rayo simply do not have the attacking quality to respond and create a goal-fest.