Greetings!
I would also like to note, on the roles of Immolators, that so long as you use a Heavy Flamer squad with them, they can move 12", fire... and then release a squad of 4 heavy weapons (and a bolter) that can still fire! Heavy Flamers (names aside) are Assault weapons, not Heavy weapons.
In addition, yes, one Immolator is likely to get roasted, but how about 5? (or 4 if you also want an Exorcist) And don't forget those HK Missles. No, they don't often hit, but who can afford to utterly ignore them?
I speak from experiance, my S.O. plays Sisters and she often wins...