Yes, the game has known issues with the AI being too good. There's changes you can make, though. There's a slider in the options if you choose "custom" for the AI's level. I think it goes from 0 to 1, and we've found .4 seems to be a good spot.
Alternatively there's a .cfg file you can alter where you'll see their abilities broken down into 2-3 stats. The important one is precision, I think, as that affects how well they aim.
You can also grab mods to help this stuff, although generally speaking they don't affect older missions in that case, just new ones made with the mod in use.
At default settings, though, the AI is like fighting Jedi with guns...they see things before they happen, and know where you are as soon as you fire! Not just firing your gun, either. It's a more global thing, so setting off a satchel charge will let them see where you are as surely as if you'd fired an RPG with a big smoke trail, even though realistically that can't happen. It's like a "IF 'player' fires THEN 'AI' know location at time of fire" regardless of weapon used. Darkness and shrubs don't block their view either, again unless you use a mod to change that.
That's the beauty of the game, more mods than you can shake a dead cat's stick at.