When I look at the matchups I think that the most difficult to predict result can the Napoli v Barcelona and Inter v Atlético Madrid. I mean theory there are favourite of this matches which is Barcelona and Inter. You know Barcelona are having really tough days and Xavi even announced his parting ways with Barcelona this summer. Napoli could make a surprise. Also, Atletico Madrid plays very consistently in Laliga as well as Inter Milan in Serie A, so these two clubs will play carefully, aggressive and defensive, play both of them are equally good.
Yes, I agree with you, for the four teams you mentioned above it will be very difficult who will qualify for the next round. Of course, the coach has studied what techniques his opponent uses and who must be closely guarded, of course the cohesiveness of each line must be maintained in the future match. If you look at Barcelona's chances, they are still strong enough to advance to the next round.