That's actually how it is in Town of Salem. Persephone has pretty much stayed true to the original ratios.
 well, with a caveat: there is also usually a Serial Killer running around, which can serve as a check to the Mafia team as he can kill Mafia as well as townies.
I was debating whether or not to add that role in, actually. But thought since it's the first time trying it on the forums, I didn't want things to get too crazy.
Also, there's usually an ability for the bad guys to convert one of the good guys. However, that's not in play here.
And Remi's right, as far as I can recall. The neutral roles that are in play have their own win condition. The Executioner wants to get their target lynched, the Survivor just wants to live until the end of the game, and the Jester wants to be lynched at any cost.
The Executioner and Survivor win regardless of who actually wins; townies or mafia, so long as they fulfill their win conditions. It's up to those people to decide whether they want to side with the townies or the mafia.