Im pretty certain blizz wont add any immune encounters any more.
I haven't seen a mob that is immune to any school and am pretty sure this mechanic is gone -
ie fire elementas are not immune to fire.
As for fire pvp its fine. Problem is u need a really high crit stat and with the current gear its too low.
Thats why frost is good atm it suits the low gear/mana pools alot better then the other 2.
And fire is far from squishy actually, the mana shield is now pretty useful, the current state of ice barrier is hilarious it absorbs
not more then 6-7 k total damage with glyph.
Advantage of frost over fire is it has 1 million snares, where fire compensates with stuns and very gear/crit dependent rng procs, while frost rng isn't gear dependent and can be somewhat controlled so it aint so random.