To me this match between arsenal vs Porto is really going to be a very difficult match for arsenal irrespective of the fact that they are playing at home this time around, just look at the statistics of the previous match at the home stadium of fc Porto, arsenal were the dominant team all through, but they were unable to register even a single shot on target, which is not a good thing for a top quality team as arsenal.
I sees Porto as that team that will makes you pay dearly if they are being underated, and arsenal need to be more direct in their attacking play if they don't want the same scenario that took place last time to repeat itself, but as it stands, the tie is still open, it can still go either ways.
This UCL season is full of surprises, don't underestimate your opponents, Munich lost against Napoli, Arsenal also lost against Porto. as you said, Arsenal were indeed favorites, even in the first leg they had superior possession of the ball but there was not a single shot on target. So it could be said that Arsenal are still ahead in the second leg, but Porto could provide a surprise. It is very important for Arsenal to never underestimate their opponents if they want to advance to the next round. Moreover, it is predicted that Porto will play defensively and only rely on fast counter attacks. If Arsenal don't learn from the first leg, it is not impossible that the same thing will happen again in the second leg.
I always believe that the UCL league is very different from the domestic league, in the UCL league what is needed is not only strategy but mentality and having the DNA of the UCL league. Currently, the mentality and DNA of the UCL league only belong to Porto, so the second leg for Arsenal will be very tough even though they are playing at home. However, if they make a mistake, Arsenal will be punished by Porto like in the first leg. What is certain is that this match will be intense and exciting to watch.