Playoff games are always more difficult because the responsibility is higher, any mistake can be fatal, so teams play more carefully. This has always been the case for Porto, they are very dangerous at home and I'm a little surprised to see 4.5 at Porto at home. Arsenal is considered the favorite in this match with 1.78, but I would not risk betting on Arsenal to win in this match.
There is still more than a month before the match, so it is not worth assessing the current form of the teams. Maybe during this time Arsenal will be able to improve their game, since they are now in a clear decline, the opponents were not the easiest, but this does not justify so many defeats.
Of course it will be much more difficult because you will be facing a team that is much more balanced and has almost the same level of experience. Porto is a representative who regularly enters the Champions League and of course their strength cannot be considered weak, especially if they play at home, so Arsenal needs to be aware of their ability to provide surprises. If I wanted to bet, I would prefer the match to end in a draw because after all, Porto is a team that is quite ready to play in the Champions League.
The match is still quite long and maybe both of them can prepare a lot, depending on the depth of the squad they have. When key players suffer injuries due to participating in the main league, this will actually affect the team's performance. Both coaches will have to get really good at rotating because the playoffs will be much more difficult.