Bayern Munich were considered as one of best candidates to win Champions League after they purchased Harry Kane from Tottenham Hotspur until recent weeks they shown some terrible performances.
Arsenal have never been a good candidate to win Champions League and they are even far lower than Bayern Munich in tradition and quality. Porto won one Champions League with Jose Mourinho but Arsenal did not win any Champions League. They got a final match but lost to Barcelona.
The problem isn't about Harry Kane as well. Because he is currently the best performing player in the team. He has scored a lot of goals to help this team achieve titles. But he is still really unlucky about winning a league title.

Even at a team like Bayern Munich he has found the season in which Bayern Munich play worse as a team. The title streak in the Bundesliga is about to end.
Kane has become last minute hero against leipzig but his performance is still average. I can't say kane has increased his standard, but he keeps stuck in the same level like when he played at totenham. There was no significant improvement happened to him. Even though he had become a hero for bayern munich sometime but this didn't change the result if his performance was not so amazing like other bomber which is still staying in EPL like haaland. Kane was still feeling so difficult to play better, probably caused by he is still in the process learning the local language. Bayern is out of my list as a title contender caused by poor performance showed against lazio.
Winning UCL is not that easy compared with winning bundesliga title. The home game against lazio will be an easy game as long as bayern will not perform like when augburg beat it at allianz arena.
Arsenal are of course not one of number one contenders. They have a great scoring potential this season as you can see from their performance in the Premier League. But they are a very young team and it adds extra difficulty when it comes to winning a Champions League title. Consistency problems are seen clearly like in the Porto game.
I never ever put arsenal as favorite team to win UCL. The performance in the domestic league can't be used as data to measure the performance in UCL. The next game in emirates stadium will be another easy game for arsenal.
Im not putting this club as a strong contender to win UCL but seeing the positive trend that is still going on in emirate stadium made me feel arsenal will win the game with huge margins against porto.