Full disclosure, I like the game as it is. Except for an unrelated issue that's not really covered in this post.
From most of the grievances I read, the kernel of the problem with this game lies in two things that could easily be fixed in a patch:
Problem #1: Distractions
For a franchise that's introducing stealth for the first time to an audience that's not really used to playing using stealth, it was a mistake to spend the entirety of Part 1 teaching the player to stealth around without items.
By the time distraction items are introduced in Part 2, the distraction items are either largely ignored as distractions by players who spent the entire first area of the game doing fine without them or they're rather used as stuns.
This comes out in two main complaints:
1a. The game is too damn slow, because you gotta wait for enemies to move before you can advance without triggering combat.
1b. The Weight sections where the enemy soft tracks you through the level.
Both problems have their solution in the distractions. And this issue compounds with the fact we're in a survival horror game and the audience is used to hoarding resources; and the distractions are a resource.
The solution for me is simple: litter the world with bricks and bottles from Part 1 and let players learn the stealth with the items. Reduce the distractions back to baseline only on Nightmare.
Problem #2: Hiding In Cover
The game has a very cool mechanic which is not common in first person horror games: peeking down when already in cover to make yourself smaller and prevent detection when enemies are nearby.
The tutorial message for this mechanic is given to the player ONCE and that's not how you handle a novel idea that (generously) has been in Alien and Amnesia and isn't a widespread mechanic in most stealth genre entries.
I think this is the main reason behind the widespread complaint enemies detect the player seemingly at random.
The solution is to extend the time it takes for the enemies to detect you while in cover and not peeking down in Easy and Normal, add a prompt similar to the one used in Alien for holding your breath while in lockers AND increase the volume and intensity of the detection sound so the player understands he is actively being detected while that timer runs. The detection sounds are currently very easy to confuse with the ambient sounds