So it's ok if you can attack them when they do not see you, but when they attack you when you don't see them the whining starts? :P
Personally, I don't have problem with enemies attacking me, like cybie do, or in open areas. AoD is another perfect example : you deliberatly chose to have a short LOS, so handle it !
What makes it not-ok is that there is no rule defining when and why monster can shoot at you when you're supposed to be under cover.
While one can probably know all the existing LOS patterns, for anyone else, it results in a purely random factor, making you vulnerable in situations where the "rules" let you think you're protected.
When it's a former human, it's rarely a serious problem, but with a commando, revenant, or mancubus, you can get one-shot...
And just to insist on it : the problem is not enemies attacking the doomguy, or even enemies attacking from hidden areas, It's the randomness around it, making it unpredictable.
For a strategic game, we need to be able to predict what happens. If moving somewhere "might expose me... or not", then I'd better play lottery.
This kind of unpredictable behaviours is one of the reason why my current "strategy" is to camp some cover and permawait with my shotgun until the level is cleaned (which is plain boring and repetitive, but removes random factors). I got too much bad surprises with current LOS, and walking is just not as effective as waiting.