Nah, SK is very easy to play against. WW too. The mechanics in the game put them at a disadvantage to start the game. If the SK is investigated by the Sheriff then they will come up as Serial Killer instead of innocent which every other 3rd party role do. Second, the SK will kill an escort or consort if targeted by them. All the escort has to do is update their death note and if killed by the SK, then the SK is outed 9 times out of ten.
Arsonist is the strongest 3rd party role and the worst of them all against Town(I would say Pestilence is but the odds of a PB becoming a Pesti is quite rare).