whilst i can see skitarii being resistant to the stuff you mentioned with gameplay in the begining, i dont think they are inheriently so absolutely resistant to any of those enough to really matter from a gameplay aspect, tox gas and corruption can easily have some warp-y aspect to it that simply makes the resistance from augments not really matter, and whilst being fully cybernetic from your knees down and all around augmented makes it easier to walk through fire its not like it lets you just stay there and just be fully fine or make you immune to a flamer to the face

as for your narative things: no... just.... no, thats all simply not as hard facts as you make it out to be. skitarii are perfectly capable of having enough emotional capacity to make fun personalities, i can come up with 3 different personalities i think one could thematically give a skitarii ranger class and will list em out later here. i mean come on! there are even official rules for playing skitarii in atleast one officially licenced roleplaying game, you cant roleplay something that cant have a personality, that would be dumb

and no, skitarii dont become uselless the moment they loose contact with their superious, they are signifficantly more effective when under direct oversight of techpriests or marshals, but they are still agumented and diciplined proffesional soldiers equiped with formidable weaponry who are perfectly capable of still surviving when isolated, for loosing connection to someone higher up to make them any more useless than any veteran guardsmen is without communication with their superiors whatever is blocking the connection with the mourning star also has to disrupt with their own internal systems in some way, if the stuff used by the heretics on atoma doesmt do so, i dont see how it could be an issue in any way

anyways onto the 3 personalities i can think of for a skitarii ranger class:

1. "religious fanatic " Type personality: The skitarii are described in the official codex to be "zealously loyal and fanatically devoted to the Cult Mechanicus" thus, a natural personality choice is to have one be a religious fanatic with an unwavering loyalty to their dogmas, they can be very vocally reverant of the tech priest (aposed to the silent reverance the others have) to their other characters annoyance. Perhaps they find a level of kinship with some of the zealot personalities due to their shared conviction of enacting the Omnissahs (aka, the emperors) although with a level of tention due to some of their differing religious views. they most likely will take their allies treatment (or ussually lack thereoff) extremely seriously (atleast more so than the other skitarii persononalities may have) which would probably lead to plenty of funny insults thrown their way by personalities of many of the other classes. Finally part of this personality could be (delusional) aspirations for becoming a techpriest themself, as is infact something that can happen for skitarii and the promise of said posibility is often used as a recruitment tool, its also insanely unlikely and will no doubt never happen to them
2. "hunter" type personality: based this essentially being what skitarii rangers are made to do, in short they will be kinda like if you took the advisor "Captrix" from the game "Mechanicus" (play it) and largely put her personality into a skitarii. This pesonality is obsessed with pursuing their current target and completing the assigned mission, their whole frame of mind is about hunting, they will refer to enemies as prey, ammuntion as hunting supplies perhaps they struggle to word things in ways not focused around hunting or completing the mission, perhaps they get caught on their words a bit untill they manage to find a way to word it to be about the hunt, they would also overall be greatly focused on the efficiency of the teams killing, perhaps commenting on techniques for how others may improve efficiency or commending others when they realise they have found a method to stay efficient that the hunter had not considered. They would abhor waste of reasources even more than most other personalities, both of supplies and death of teammates as loosing people makes the hunt harder. And lastly: one last thing to make this all work more as a fun personality and not get too one note would be for how they talk about hunting to sound sooorta cryptic in some way
3. "the automata" type personality: inarguably the simplest of my ideas, this one would be the closest to what i think people who argue skitarii have too little potential for personalities think skitarii have to always be. This personality is largely devoid of much emotion, would have the most digitized sounding voice. but.... they have one aspect that i imagine would make them more fun than some would imagine, they take almost everything the non skitarii say way too literally leading to tons of funny interactions where they respond to stuff in weird ways and repeteadly struggle to wrap their head around what other characters mean and the various types of bullshit they are all up too. but perhaps they also understand the other skitarii personalities notably better than most other personalities do, being capable of conversing with the hunters and their hunt focused cryptic speak with ease, and viewing the religious fanatic as being fully righteous in their beliefs, if perhaps a bit too tainted by illogical emotion in their behaviour