On paper Real Madrid are far superior to Salzburg and Brest, but anything can happen as long as the match is still in progress. Although the chances for both clubs are very slim, it could be very decisive and could punish Real Madrid. What just happened at Anfield when Man United held Liverpool to a draw could happen to Real Madrid.
This match is very important for Real Madrid to determine their fate in the UCL, they must take advantage of every opportunity and prevent the slightest mistake so as not to lose points at a crucial time.
Honestly I will not underestimate RB Salzburg and Brest, moreover Madrid is still a bit unstable when playing against teams below them. However, because this is the Champions League, at least Ancelotti and his troops understand the atmosphere on the field much better. Talking about the Champions League, the most important thing is the mentality to play until the end, therefore RB Salzburg and Brest must be able to match the mentality of Real Madrid game. That is the only key that other clubs need to prepare when playing against Madrid.