Anyone remember golden eye for the n64? I remember the unlocked 007 mode where you could adjust difficulty by tweaking health, damage, accuracy and the like. One bit of fun was to rig accuracy and damage to 2000% and try to make though without dying. One shot meant death.
Now I bring this up because it represents the idea here, an opponent who can, theoretically, execute you at a large distance long before human response or knowledge of the enemy.
Back to the nostalgia. Some maps were a blast to do this in. Close quarters, places like the bunker where you could surprise and ambush, where you'd need to sneak and be careful, well this was great. Some maps, not so much. Maps that were big, that were open and that were littered with enemies with scoped weapons (looking at you siberia level). Those were not fun to play. You'd spawn, have a second or two of life then die from mystery shot of god.
To tie this all back together, this shows that it depends on the sort of game and the intent of the enemy to decide if this is good or bad. If the purpose is to force the player to use other tactics, to avoid being seen, to try to out smart or out snipe the opponents, then it can be a valid addition. It is not meant to be a run and gun, so punishing people who try to run through is a worthwhile part of the design.
On the flip side, if the game is meant to be more of an action title, where twitch trigger guns or chest high walls litter the landscape, then no, you do not want headshots by enemies. The designs of those games would make the kills cheap and the rest of the game supporting that play style only to be punished for playing it right when some AI busts out the snipers. Games don't have to be real and don't have to be fair in how enemies and players stack up. In some shooters, the headshot crit sort of mechanic would really mess with that when controlled by an enemy AI.
