You shouldn't blame anyone who is underestimating Arsenal, because they just see it from their own perspective that Arsenal might not be able to go beyond the stage. The reason is because, they are playing with a club that is more experienced than them in the Champions League. If I want to start naming one of Arsenal players that is certainly going to net the post on that match day, I cannot call any.
Arsenal for the first time after 2007/2008 have managed to reach the quarter final with a penalty shootout, which shows that it was a narrow escape, and a game of luck. However, we might think that the gunners don't have any bullet in their gun, but will take some shots on that day.
Everyone has their own point of view and everyone has speculation about the betting choices they will make. There's nothing wrong with that and I've read several views for those who support Arsenal and also those who support Bayern and I think it's quite logical but after I saw both of them I increasingly confident that these two teams have the same percentage to win the match.
So I wouldn't bet excessively on this match because anything is possible.
Arsenal currently cannot be compared to Arsenal in previous seasons, they have improved a lot and so have Bayern, many differences have occurred this season.