When creatures die from just exposure to the warp, it's usually because their hearts stop with fear, or their minds are incapable of comprehending the sheer madness they perceive. Walls melt, colors become sound and flavor, reality literally becomes unreal and frighteningly surreal. It would be like sneaking up on a sleeping new-born infant and roaring loudly directly into its ear; the shock of the stimulus could cause heart failure, certainly a lasting trauma caused by fear of the unknown or unexpected. Even with a Gellar Field in place, people that have looked upon the Warp have gone blind, eaten their own fingers off, or completely lost their minds, becoming catatonic.
In many GW, BL, and FFG passages you can read about how Tyranids seem inexplicably immune to the effects of the Warp. That's not to say that Warp energy couldn't boil them to explosion, or abrade their flesh from their bones, or crush them into tiny balls of super dense mush. Genestealers are the most common creatures specifically referred to in these passages. They are also completely immune to exposure to the void, have no need to breathe, are immune to disease and almost every natural toxin they encounter, and can willingly enter a suspended animation/hibernation. One person on an expedition into a Space Hulk that gets implanted by a Genestealer is all it takes to bring utter ruin to an entire planet. And it only takes one Genestealer to start the whole tower crumbling. Being immune to the pandering promises of Chaos entities and the babbling lesser beings makes hibernation a little less noisy.
Now, regarding the Warp and Genestealers…what would happen, say, if a creature that were gestating a 4th-gen, or Pure-strain Genestealer, but prior to spawning the vile thing was exposed to the mutating effects of the Warp? Would the Warp then somehow mutate the embryonic Pure-strain, through the mutation of its host, possibly causing the birth of a Warp-mutated Genestealer? What would that creature be able to do over and above that of its "normal" kin?
I think Tyranids are as inscrutable to Chaos as Chaos is to humanity. There's really no "mind" there for the Ruinous Powers to manipulate, control, or turn toward their adulation. The Hive Mind has one overriding impulse, one objective, one imperative…devour. Chaos' agendas are divided among different spheres, and none of them really approach their areas of interest and influence with quite the singularity of purpose as the Tyranids.