Ah yes, creatures and their shoddy perception.
Yes, that is why is that, alot of creatures have really shitty eyesight (or non at all) when compared to Humans so you can pepper them with lead with impunity.
Alot of them have "psi sense" which could be interpreted as them being able to smell or sense in some way your troops.
If i look (UFO pedia) at the standard spider, it has psi sense of 15, so its spotting distance is 15 titles if they turn in your direction. Human troops can see way beyond that distance and with things like blackops rifles you can easily shoot across the map via aimed shot.
Alot of creatures have this and the whole gameplay around them is based around your troops maintaining distance while picking them off one by one, ocassionally retreating if they get too close. And creatures really hit hard, hell, something like a Werewolf can destroy your tank if he gets in melee. And high numbers can be problem too. It kinda sucks if your craft lands in a middle of a city with +33 scorpions scattered in every building so its impossible to tear them to pieces during your own turn and you need to use reaction fire.
There are few creatures with ranged attack so dont get too comfy, but they are rare or their damage can be countered with non-end game armor.
Use creature missions to train your troops, thats the purpose i use it for since beyond helicopter craft it has become impossible to lose.
And when you get flying suits, then its just a training exercise while your troops can shoot them with impunity.
Edit: There is an option for sneaky AI in advanced options, but i cannot fathom maintaining my sanity with it. It can be infuriating hunting one thing in a jungle that doesnt even try to hide.