Napoli will not be so difficult to face Frankfurt in the Champions League this season even though what you say is true too, but Napoli on the other hand also has the ability to dismantle any team's defense this season. Even though the two of them have yet to meet in the Champions League this season, I see that in Serie A Napoli have often never made a fatal mistake this season so that has made Napoli look more convincing to win against Frankfurt in the Champions League this season.
If we talking which club will win, obviously it will be Napoli.
But if we talk about to bet on the first leg and second leg, I will give advice to bet draw on the first leg since it will be played on Frankfurt's home, while the second leg it will be played on Napoli's home and I will bet Napoli to win. Frankfurt currently at 4th position on Germany Bundesliga, not a good achievement. The second position is Freiburg who haven't lost any single match on Europa League, and the third is Leipzig who can beat Real Madrid. It's lucky for Frankfurt to play on Champions League.
I didn't counted Frankfurt as the team who qualified to round of 16 before because until week 4 Frankfurt position at the bottom of standings group but surprisingly they can able to beat Sporting on the last match and makes them finish at 2nd position below to Tottenham so this team had capability to makes strong teams to slipped so i personally will very wary to Frankfurt although on the paper Napoli is the favourite but this team is hard to defeated and on the first leg i am sure it will be utilized by Frankfurt to gets 3 points and what Napoli needs to do is try not to lose that if Napoli can able to steal point in the first leg i am sure second leg will be more easy for them