Its not so much that there are many lightning immunes, its that lightning immunes are your bottleneck and it doesn't matter how fast you kill non-immunes if you are obligated to also kill immunes, otherwise you're just skipping kills.
Still I think you have to compare it to an equal fire sorc, who probably has the upper hand with renewed sunders and heralds in the game now.
105 Fire sorc hits about 17.3k fb / 35k meteor at 8 fpa with -93% pierce on gear (+conviction+sunder), and the 4200 hydras without cooldown or nextdelay are now 55.4k dps when fully deployed, about doubling the fireball spam dps
There's a shortcoming to each. Nova and lightning can both use ES, fire has to be vita or drop hydra and use massive prebuff swap which nobody bothers in pvm.
Nova has suicidally short range and lower dps in exchange for clearing trash faster. Lightning has a long piercing aoe but slower animation. Fire has the highest focused DPS but small aoe and delays to set up.
When you're just killing champions and heralds and bosses and mostly ignoring trash, fire seems like it has the most upside. FB alone kills champions, 6 hydras-> meteor+fb just destroys herald packs that nova can't even approach
Light immunes aren't the bottleneck they once were with this extra pierce. I think fb/hydra smashes any flavor of nova if you're pack hunting in stony tomb or herald hunting. Pack hunting in stony field or full clearing flayer jungle? 200 fcr nova takes the cake. Tombs? Probably self infi wins there. Pack hunting chaos? I think 200 and 105 nova are tied and outclass any fire build there.
Btw I use ES even with 1 pt ES, 1 pt telekinesis on fire sorc, and go half life half mana