Last season Napoli also played well and they also led the standings and competed with several other top teams, but in the end they failed to maintain the consistency that made Milan win the Scudetto at the end of the season. I don't know what the scenario will be this season, but Napoli are one of the favorites in Serie A which I might agree with.
This has happened for several seasons and the problem is still the same, they lose consistency when the season is running in the middle of the half.
At the moment they are still in very good condition and of course it is very good but there are some big games they have to go through now because not only in the Champions League they will also face some big games in Serie A later including Liverpool in the Champions League and Atalanta, Udinese and Inter in Serie A.