Porto was of strong team. Even though we are rarely seeing the performance of porto in its domestic league but it's still competing for the top place in the portugal league. Arsenal shall never try to underestimate its opponent. It can make the players become over confidence and they will be starting to bottling the important match against porto. This is the bad habit owner by arsenal as it was always failing to go to the further stage of UCL. Arteta shall also aware if this is the main problem that needs to be solved.
He needs to push his player to perform even better compared to the previous game. Arsenal was getting easy team in the group stage but this time is kinda different. It's not facing a weak team. Arsenal needs to be careful if the team is willing to win the game.
Im not counting on arsenal caused by porto and arsenal are same strong. Neither of them will be dominating the game soon.
Historically they are definitely a great team there is nothing wrong with saying that, but to say that they are still a strong team, specially in the context of UCL, is not really fair. There are teams willing to spend more than 10x what Porto could ever spend, so it is not really fair to say that they are good, nor it's fair to say they are "bad", they are as good as they can get with their financial situation.
A premier league club would make more from finishing 10th than the winner if Portugal league, so it is normal for Porto to be not that great as a UCL winner type of team, but they are doing the very best they can with the financial situation that they are in. I believe that they will continue to get better and better.