Also, excuse my 3rd post, but if you are going to have a separate category for Cult, SK, masons.... then you might as well go for Cop, Doctor and some other common role. While you're at it, why not split the Mafia into 'Vanilla Mafia', 'Mafia Godfather', Mafia this, Mafia that, you'll never end.
Splitting roles into three categories is simple, I think it works, though some roles are indeed hard to categorize. The goal is simply to have an idea how players performs with different roles (Mafia/Neutral/Town), but you really don't need to be specific. Three categories is just fine, and the real statistic, the one that really counts, is how many games you won/lost overall (all roles included).