Before the get go just want to make it clear I understand "difficulty" is subjective. I'm under no illusion that I am a expert marksman in this game. That being said, I still have a few stipulations..
The AI detection is madly insane. I do give the game development credit however on the accuracy of the enemy AI depending on there affiliation or background. The shooters in Elephant for example feel in place as they are not ex military or some high paid mercenary. Just crazy kids who picked up guns. But then you have the enemy AI who apparently can see as far as a sky drone.
The human eyes can only pick up movement so fast. It's challenging if its realistically possible. But when you have enemy AI doing the impossible, your success feels more like dumb luck than personal achievement. When inside buildings it becomes a bit more "fair" for sure. That and you get the ability to use tools in your arsenal to disorient hostiles. When outside however, or in very very large environments between the boxes, plants, cars, random debris, the random forklift or whatever else is in your way. Your eyes are up against an opponent that knows your there the moment your elbow is sticking out from any corner.
Maybe its just me. or maybe many of you will agree. Maybe the difficulty was always intended to be like this so replay ability is plentiful. Please mind I'm basing all of this if going for S rank. There's a difference between just going on a murder spree vs getting that S rank where one death regardless of which side your on results in failure to obtain the S rank.
Thanks for your time.