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.
Almost all English league clubs refuse Ronaldo to be brought in maybe because they know how Ronaldo's attitude will be an obstacle later so that many are ostracized and not on the radar of English club supervision this winter.
Of course attitude is important especially Ronaldo should be able to set an example to others that attitude in football is not like that if they don't play well and have to accept the reality if they are substituted, but Napoli are really interested in Ronaldo even though we know the Napoli squad is on fire but maybe Spalletti wants to need Ronaldo's services as long as he changes his bad attitude for the better.
They worry that if Ronaldo comes, he will break their standards in salary and relationships between a coach and a player. Generally, they see more risk, more troubles than benefit and they want to avoid it at beginning.
Prevention is better than cure and they want to prevent it rather than to resolve troubles and conflicts started by Ronaldo.