The champions league is a very competitive league where all teams come prepared for that tournament. The underdogs in some other domestic leagues may still perform far better than the best teams in other leagues and could even eliminate them if not played with caution with them.
Champions League is a league for biggest and richest clubs so it has very limited opportunities for small clubs to be successful underdogs then become champions like Porto in 2003 - 2004 seasons, twenty years ago.
Napoli in the past season is an underdog but they had to stop at the quarter final and they will likely not be able to repeat that impressive result in a new season. With small and underdog clubs, going through group stage and appear in quarter finals like Napoli is already huge success for them.
Manchester City might be the best team last season, but this season they might likely not be able to go far as the previous season as more eyes are already on them and every team coming to play them would have trained extra just to defeat the defending champions.
Manchester City only shown their superb strength in a second leg semi final with Real Madrid at Etihad when they did not give Real Madrid any chance to control ball and make dangerous counter attack. They were not able to do that in a first leg semi final at Bernabeu. In the final match against Inter Milan, neutrally they did not win it by dominating that final and Inter Milan brought lot of challenge and dangerous chances to score goals.
Manchester City won Champions League in the last season but they have far away from such domination of Barcelona and Real Madrid years ago. A new season will be another brick for Manchester City to perform and convince us that they are actually becoming a new giant in Champions League and will be able to dominate it in coming years.