Survivors have no alignment to anyone but themselves, and win if either the town or the Mafia win, and they are alive at the end as well. Survivors are one of the harder roles to win as, especially in larger games, as they usually have no way to avoid nightkills. However, survivors are generally only lynched if the town views them as a threat to winning or if their survivor claim isn't believed, as lynching survivors does not advance the town's win condition but does advance the Mafia's.
Because survivors are self-aligned, they can potentially be a threat to the town, especially in later stages of the game. Survivors can potentially bring about LYLO earlier than normal, since they can throw their vote in with the Mafia, adding an extra vote to an endgame bandwagon. However, this concern is not entirely realistic; the survivor would need to know it is lylo and also know he or she is not voting for a mafia member. Such a scenario is unlikely to occur.
At best, survivors are simply extraneous players that can be ignored. At worst, they take their Survivor win condition to heart and spend the entire game constantly flip flopping between which faction should win, scum or town. In lylo, a Survivor may completely hand the game to the scum if he or she determines that this is the best course of action for him or her to win the game. You should never trust Survivors.
Very rarely there have been cases of pro-town survivors; players who only win if they are alive at endgame, AND if the town is the winner. This makes them the equivalent of normal townspeople, with the exception that they really don't want to die.
They are controversial, and have only been used once or twice in the past.