Yes, Spalletti failed with the Italian national team after leaving Napoli, he was one step away from not making it to the World Cup (Italian fans would probably go crazy from missing out on another World Cup) and there was also a failure at the European Championship. It seems that Napoli was his peak. As for Conte, in my opinion, he shows results higher than expected and if he progresses, Napoli will have a chance for another title.
Being a national team coach is harder because they are rarely together and only at certain times they can gather. Even though they can choose any player they want, the lack of time together is still an obstacle.
Spalletti's decision to choose Italy over Napoli I must admit was a brave decision and also full of risk. Because he left when he successfully brought Napoli to become champions. But that was his decision and it was indeed one of his wishes even though it did not go as he wanted.