Ach Naples is totally fine. Just don't wander too far from the city centre at night (I mean go into the suburbs where tourists normally don't anyway) and you'll be fine. Just like any other city keep your wits about you.
Thoroughly recommend this underground tour. Best to get tickets in advance when you can as the queue can be massive, or get lucky at the end of the day!
https://www.napolisotterranea.org/
Also if you like to try different beers, go to Versa. Just google map it, very central. Tiny bar, free popcorn, absolutely brilliant bar staff who were so great we ended up staying until closing, which isn't like me and the wife to do nowadays.