I've looked around on HB out of curiosity of what people know about this topic, but have not seen a significant amount of discussion compared to lighter roasts seen more commonplace in the US. Much of this discussion also seems to be speculation as well, and I believe I have knowledge that may be able to clean up any questions or notions.
A quick summary of my experience: I am an American who has had the privilege of completing my last two years of high school in Campania Italy and am now in my third year of University. I go back and forth between the US and Italy during this time, spending on average 5-6 months a year in Naples area. I got into coffee during this time and am now a certified Italian barista. My favorite roasts are local to Campania, and would be considered Neapolitan to me.
I will start with the definition. What is a Neapolitan Roast? I consider it to be a darker roast to American and even Northern Italy standards that uses an Arabica and Robusta mix to create flavor notes that dominate with caramel, vanilla, chocolate, and nuts. The percentages can range from as low as 45% arabica 55% robusta to some roasts that use only arabica mixes. My personal favorite roast uses a 70% arabica and 30% robusta miscela from a relatively small roasting company employing 5 people total. Mono origin coffee is known, and specialty roasted for clients, but the majority of the population does not drink them.The body of the shot is meant to be almost very thick if not syrupy with a pleasant aftertaste with a mirror taste to the shot that is meant to last 15-30 minutes if properly extracted. This thicker body stems from the robusta bean, which does produce more crema than arabica and also has a higher caffeine content. Robusta is seen as a cheaper, lower quality bean than the arabica bean, but I believe it has merit to complement the arabica in the many 'miscele' seen and created by the torrefazioni in Campania.
In Campania, there are many coffee roasting companies (approaching 35), ranging from KIMBO S.P.A., down to the prior mentioned 5-person company Saka Caffè. The largest players may or may not be known, but the 5 largest are
Caffè Toraldo S.R.L
Caffè Moreno S.R.L
Passalacqua S.P.A (probably the better known company here in HB besides Kimbo)
Caffè Kenon, L'oro di Napoli
Every bar uses one of the many miscele of many companies, but the general setup of the bars is consistently homogeneous in nature. The majority of roasts can be seen as similar in notes, but the slight nuances will win over each loyal customer. Of all provinces in Italy, Campania generally sees the most lever machines in use. In the downtown historical center, there is surprisingly a large number of the local Neapolitan Espresso Machine company Boscos considering their cost.
Choosing a specific roast is only half the result of what you drink at the counter. The skill of the barista and the cleanliness of the machine generally comprise of the other half. A poorly maintained machine/grinder and an inept barista will ruin a shot universally, and Campania is not exception. Bad bars can be meters from a great bar, and simply looking into the bar will not tell you the quality of the espresso produced. A bar unchanged from the 70's or 80's may be hiding one of the best tasting shots in Naples, as I have found time and again. The example I use here is a 5 minute walk from the airport that I passed on for years due to its shabby appearance. I decided to try it after 4 years, and was completely floored by its quality. They were using caffè moreno for reference. The outside of this bar is shown below.
I can delve more into the subject, but first I would like the opinion of others here and ask how you view a Neapolitan Espresso. My personal thoughts is that this espresso is very earthy, humble, and inexpensive, similar to the Neapolitans themselves.