Bayern Munich is in an upward trend recently, and while they don't always play good football it is evident that Arsenal is not that great on the UCL. Needing to win on extra time against Porto shows that they can struggle against team that defend well. Against City and Liverpool, they managed to win due to defensive errors iirc, so they need a little bit luck if they want to do the same thing against Bayern.
Lol, I really don't understand what you meant by defensive error. Do you mean Arsenal taking 4 points from Liverpool both home and away is a defensive error? Even beating Man City on penalties to win the Community Shield and taking 3 points in the EPL is also a defensive error?

. In the rules of football, 'if you don't take your chances, your opponent will torment you.'
My question: Is error not part of football? Arsenal choosing not to make such errors shows that they were the better side. Besides, Arsenal even gifted Liverpool a goal during their away game, which means Liverpool never scored any.
When a team plays defensive football, it's very difficult for them to transition to attacking football, which means their backline is never exposed and their frontline lacks momentum. Don't forget that Bayern Munich lost to Lazio 1-0 in the first leg, and Leipzig gave Madrid a tough time in the second leg, making Madrid look like a local club.
Inter Milan happens to be the only team that plays good attacking and defensive football, but sadly, they are out. Have you not wondered why City concede so much even with their good team? They choose to play more attacking football, thereby risking their backline. But I think Arsenal knows how to keep every position steady more in the Premier League, which means they have a lot to offer in this Champions League.