all three of these mechanics can be good if done right, but therein lies the problem, i can't comment on cover based shooting or health regeneration so i'l give my 2 cents on auto aim
the best example i can think of for a well done auto aim system in a console shooter was the syphon filter series (yes i know its a TPS and not a FPS but please bear with me) the auto aim system in it wasn't on all the time, you had to hold down a shoulder button in which it roughly locked onto the nearest enemy (but you could switch between targets). Great for blind firing and shooting on the move, but not much good for ammo conservation, or if said enemy had body armour, and alternatively if you wanted to go for precision shooting you held down a different shoulder button to go first person aiming
the best example i can think of for a well done auto aim system in a console shooter was the syphon filter series (yes i know its a TPS and not a FPS but please bear with me) the auto aim system in it wasn't on all the time, you had to hold down a shoulder button in which it roughly locked onto the nearest enemy (but you could switch between targets). Great for blind firing and shooting on the move, but not much good for ammo conservation, or if said enemy had body armour, and alternatively if you wanted to go for precision shooting you held down a different shoulder button to go first person aiming