I am having an absolute blast with this game but I must confess that my playstyle basically revolves around gaming the (rather stupid) AI, as opposed to behaving as I'd imagine a real sniper would.
As far as I can tell there are simply far too many enemies to ever take them all out 'properly', so what I tend to do is tag up as many as I can in an area with binoculars (where possible, often times they're indoors), find myself a decent vantage point, and then start blowing up their heads/organs/testicles with gay abandon, caring little for an ALERT status, and not even too much for COVER BLOWN either, depending on how well they can path to me.
On the level I played last night I was having a bit of a trouble with a large factory complex stuffed to the rafters with Nazis, until I found a high roof I could climb to which didn't have any other direct connections to any other building, so the only DIRECT way to the roof was up the drainpipes I'd climbed up to get there. (There was a jump from a walkway just below the roof to a walkway on an adjacent building, which I could use with ease, but the AI didn't seem to recognise that as a traverseable path so it never attempted to use it to get onto the roof.)
The AI pathing had no way of coping with this, so even when COVER BLOWN status was achieved, all the baddies would just run around trying to take positions and look like they were flanking, but since they were unwilling to climb up the drainpipes (probably didn't want to ruin their nice crimped Nazi uniforms), they literally had no way of getting to me. As I could retreat enough along the roofline so that none of them could line of sight me, after a short time I'd drop down to ALERT status and not long after that they'd all give up and go back to their idle (albeit evil) Nazi chit chat.
(I have attached a splendid top-down diagram to illustrate.)
Now admittedly that was slightly unusual in that generally speaking the AI will have a route to get to you, but even then hiding behind a rocky outcrop and picking them off as they run at you often works well, or just running to a nearby area of cover will be sufficient, and repeating the process again. It has occasionally surprised me with how far they'll fan out from the COVER BLOWN marker, and also bring in extra baddies, but once the beats of the game have been learned, it's fairly easy to work around.
To be clear this isn't a complaint, IMO this is the way all videogames work, not just stealth/sneaky/snipey style games. I mean, I don't think we even WANT super-clevel AI in videogames, as that'd just be incredibly irritating to play against and wouldn't make for a fun game. (Two of my favourite games ever are Dishonored 1 and 2, and again, a lot of that was learning the limitations of the AI and how to play against its abilities and weaknesses.)
Maybe I wouldn't get away with this on higher skill levels, although I can't see what the AI would be able to do differently in the kind of situation outlined above, unless its pathing ability fundamentally changes or improves.
Anyway, SE4 is the bomb and I'm loving it to pieces, will deffo buy the campaign DLC when I've finished the main campaign, and then probably get myself SE3 and do that.
(I've already bought the silent weapons DLC for SE4, to try and do a whole level without getting ALERT or COVER BLOWN, as that feels more snipery.)