If you close your eyes and manage to forget everything that they have informed you about where you are and what to expect it to sound like, and just listen without either focusing on any sound in particular nor ignoring any other sounds in particular, and it sounds really good to you without any annoying elements distracting your attention, then a pair omnis properly arranged will make a more natural sounding recording from that position than a pair of microphones with any other pickup pattern.
I've made fantastic recordings using omnis from places than many folks would say they should never be used.
Instead of a list of where to use them or where not to, consider the attributes which make omnidirectional microphones different than other patterns, and use that to decide when and where their use is appropriate:
>They pick up sound equally from all directions, which can be good, bad, or both at the same time. It is why they sound very natural, and open, and is why they'll pick up the sound of idiots talking behind you, or annoying reflected sound bouncing off the metal roof of a concert shed, or the rear concrete wall.
>They are considerably less susceptible to both wind noise and "handling noise" (vibration, movement, shaking, etc) than any other pattern.
>Cost wise, they generally produce better quality sound per dollar than any other pattern.
>They are easier to setup and use without problems. Setup configuration specifics are less critical than with directional mics.
>They are more forgiving of slight movement or rotation while recording without producing audible problems.
>They "go lower" picking up the lowest bass, which can be good or bad.
>No proximity effect- their tone doesn't change at close recording distances.
>If you mount them close to something not acoustically transparent (a wall, your head, a Jecklin disk, a ping-pong paddle) they become semi-directional.