Nice to see a lot of people feel Stalker and its offspring are pure evil in terms of difficulty; I recently played through Call of Pripyat (not finished it yet) and was feeling it was a bit too easy... till I met the first Monolith fighters in the Pripyat 1 tunnel; was headshotted by a monolith fighter wielding and SVU and then next life, took an SVD round to the torso and almost bled out because of extreme blood loss; the fact you bleed makes Stalker that ever bit harder.
And the fact that half the enemies refuse to die until they have received around 200 rounds in their bodies doesn't help; I took four, FOUR RPG-7 shells to kill the pseudogiant in the Gauss Rifle lab; they are hard enough to find, but I had to burn through four of them to kill the thing!
Add the fact that despite having RPG elements, you have practically no carry weight and need to eat to prevent stat deterioration and can absorb and keep nasty radiation and need protection from thermal, electical and psychic threats as well; sure a lot of armour have said defences, but rarely enough to prevent, say a Controller, from seriously messing with you.
In the end, the Stalker games (SoC and CoP anyway, never played CS) are IMO ones that wants you to suffer and I do wonder if anyone has ever completed a playthrough on the harder difficulties without dying; even on Rookie the first time I played SoC, I died a dozen times before even reaching the 100 Rads and CoP on Normal is pretty brutal.