I said Mussorgsky was a romantic nationalist (Russia), not a romantic in a Schumann-sense. Sure, his penchant for the declamation of his language and his bold voice-leading and inspiration made him a pioneer, but in the grand scheme of things, 'Pictures at an Exhibition', 'Night on Bald Mountain' and 'Boris', all belong in the grand and supremely varied Romantic school. It just isn't Tchaikovsky.
I listened to some 'Poltergeist' today for I wanted to see whether I remembered it correctly and, like I said, the score ranges from happy-lullaby tonal, to angry-anxious chromatic, but still very much TONAL. Don't let some discords or quasi sound-mass effects throw you. And 'Atonality' as a term is not any more inappropriate than the terms 'Neo-Classical', 'Modern', 'Spectral', etc. All conventions that would be thrown out the window of any proper scientific university school save for the deliberately vague, all-embracing one of Humanities (as opposed to what?...)