Worrying about troubles with Ronaldo and his attitude is one of biggest barriers for any club want to sign with Ronaldo. They see more risk, more troubles with Ronaldo than what they can receive from him.
Ronaldo must change his attitude and accept to have a flexible role in any club. Sometimes if he plays well, he will be in the squad and get important position but if he plays bad, he must accept a position on bench. This will make him a more useful player than a troubling one.
The sentiment in Napoli is very positive and good, hence they don't want to have a troubling player like Ronaldo. They must completely focus to win their first Scudetto after many decades. Any unnecessary trouble is not wanted by Napoli and Spaletti. It is their most priority, not about salary or transfer cost for any player.
Don't demonize Ronaldo. His troubles at United were mostly due to promises from the club and Ten Hag himself that he was counted on and needed by the team. And immediately from the start he was put on the bench. What is this if not a deception and why should Ronaldo not be outraged? And in general, I remind you that Ronaldo has a huge salary - many players in his place would not give a damn about everything and would receive such a salary without playing and without worrying. But Ronaldo has ambition and wants to play. I think it deserves respect.
I think that if Napoli agrees with him and fulfills its agreements, then he will not bring any problems to the club, but rather make his contribution.