That's it; any club can lose in the Champions League. While PSG's league play last season was truly outstanding, when it comes Champions League their stand was not nearly as strong even with Messi and Neymar remaining on the team. With people still underrating Manchester United based on their early-season performance, that they will be defeated by Bayern Munich, I believe that even the clubs that are not that good in their league but qualified for the Champions League will be very serious with any team they faced. Munich is a very excellent club, and when it comes to the Champions League, they usually take their games seriously. We shouldn't consider that this season would be any different because, in my opinion, they don't have a formal coach.
Since Arsenal has been performing well this early of season, I believe that they will advance to the next round. I don't think they can lose to any team during the group stage.
Actually, Arsenal is in a safe group position compared to other clubs. So of course so far the only one who can give Arsenal a bit of resistance is Sevilla and both of them are certain to qualify from the group stage. However, after that it will be difficult to predict because the clubs that qualify will be much more difficult for Arsenal to face, even with a performance that is considered quite good this season. As we know about PSG, without Messi and Neymar, stability would be easier to see and the attacks would be more even. Mbappe on the left, Dembele on the right and Asensio in the middle is an ideal combination. For Bayern Munich, I don't think there will be any problems in the group stage or afterwards, with Kane as the sharp spearhead they will change to be more aggressive than last season.