Neapolitan polpette are not a dry meatball: they are simmered until tender in a long-cooked tomato ragù, and that sauce is the problem the sandwich exists to solve. Spooned into bread with their gravy still on them, the polpette napoletane are a finished wet dish asked to behave like a filling. The meatballs themselves are soft, bound with bread soaked in milk, often carrying raisins and pine nuts and a little grated cheese in the Neapolitan way, and they arrive glossed in a red sauce that wants to go everywhere. The whole craft of this panino is moisture control, because an unmanaged spoonful destroys the bread before the last bite.
The technique is reducing the sauce and choosing a bread that can take what is left. The ragù is drained back or cooked down thick enough to cling to the meatballs rather than pool in the crumb; the polpette are sometimes lightly crushed so they sit flat and stable instead of rolling out of the roll. The bread is taken sturdy, a firm roll with a real crust, and is frequently toasted so the inside firms into a wall the gravy soaks slowly rather than dissolves at once. The portion is controlled on purpose, two or three meatballs and only the sauce that stays with them, because the entire idea is a Sunday plate made carriable, and an overfilled one fails at the seam and stops being a sandwich.
Naples and the wider South spoon several of these long-braised dishes into bread, and each is its own subject rather than a version of this one. There is the parmigiana di melanzane layered into a roll, the salsiccia e friarielli with its sausage and bitter greens, the polpette al sugo of other regions cooked drier, and the modern hand that fries the meatball for crust before saucing it. Each is a different wet Neapolitan dish given a handle, and each deserves its own article rather than being crowded in here.