It may be quite surprising with the result, but if we look at the statistics, Arsenal really is not that good this time (except Raya).
Arsenal are fortunate to have a goalkeeper as great as Raya who was able to ward off an Atalanta player's penalty kick and followed it up with a classy save when blocking the rebound. If it wasn't for him who performed brilliantly under the crossbar, perhaps Arsenal would have lost at Atalanta headquarters. Yes, maybe Arteta prefers to play cautiously to prepare his team for another Big Match against Manchester City in the EPL. Arsenal back line plays very well but was not matched by the midfielder and front lines, perhaps this is a lesson for Arteta who does not want to bring in a new striker at the start of the season.
Indeed, Monaco is a top team in the French league, of course that's why they entered the UCL. However, to be honest, sorry if compared to Barcelona, their team strength should not be balanced. Barcelona should be superior in all lines. However, unfortunately, it is not because Monaco is very strong, but because Barcelona only played with 10 players from the beginning of the match. It was really difficult, 10 vs 11.
As I wrote in a previous comment, Monaco strength is very difficult to predict because they have not qualified for the UCL for a long time. Monaco were lucky in this game because Barcelona had to play with 10 men and a fatal mistake was made by Ter Stegen, if both teams played 11 vs 11, maybe the result would be different.