Ronaldo can bring more confidence for Napoli players. Also, italian league a bit have a slow playing rythm, so if Ronaldo join to Napoli, he can give a chance to bring trophy since with his skill and his performance, still applicable for Serie-A. Don't forget about money, merchandise sale can help their finance. The only problem is his salary, i'm sure maybe hard for naples to pay him.
Ronaldo can bring more confidence to Napoli players if results are good but if results are bad, he will create more pessimistic sentiment and more pressure to Napoli coach and players.
He is different than Ibrahimovic because he cares more about himself and his personal records first. Ibrahimovic cares more about clubs he plays for. Ibrahimovic is not obsessed with personal record and personal achievements. Ibra does not create negative pressure to his team mates and his coach like Ronaldo.