In order to have a "scary" horror game, realism could be a good choice - otherwise, the players might fall back to fantasy tropes of peasants slaying giants.
It happened to me in one D&D game. I describe a monstrous entity and the fighter said "I attack". I said, "well, it is the size of a bus...", he attacked anyway, dealt 8 damage or whatever, and got ignored by the entity.
His actions were not absurd in a game where you're supposed to fight dragons.
"Light" rules are important too. If the rules are complex enough that make you study the character sheet for solutions, you disengage from the "fictional" and therefore lose any "scare value" that you could get from it - as described in the example above. The players was thinking "attack bonus" instead of "giant-sized flying worm".
In an horror game, you should spend significant amounts of time engaging with the fictional world - building tension, describing clues, etc.
I remember one CoC game that got my players scared by simply describing an empty room with some green ooze under the bed.
At this point, "I roll perception" could ruin the mood.
But if you need this "roll perception" stuff, the system should be quick and deadly about it.
Unknown Armies has the right idea about this - PC skills are often around 20% to 40%. They are unreliable and fail more often than not. Combat (especially with guns) is extremely swingy - you can die with a single shot OR after more than a dozen stab wounds.