We're already being told that this is a masterpiece. The internet is saying so, the kind of people who like to say these things are saying so; it's easy to get carried away. Perhaps because so few aim as high as Animal Collective, or have such power to sooth and surprise, it's tempting to forgive them their slips and their sloppiness, and file them next to Smile. Well, yes, much of this record is wonderful. But let's keep a sense of proportion.

In a recent chat with The Quietus, the Collective outlined their objection to “the loudness war”, the ramping up of virtual volume by producers and engineers, convinced that ever more comical levels of compression are the only way you'll notice that the record is there. In practice, this means sacrificing subtlety for scale, flattening peaks and filling in troughs; sculpting a slab. This basic misreading of how music works - along with the continuing decline in importance of the arrangement – has effectively choked much modern music, closed down the space in which the magic happened, and the result is this generation of rock and pop records as dense and impenetrable as white dwarves. Stepping into Merriweather Post Pavilion, it seems at first that Animal Collective have learned these lessons well. A truly fantastic opening track, 'In The Flowers' wanders out of a mist of echo, uncertain chord changes picking an uncertain path. When, two and a half minutes in, it steps on a landmine, the upward blast of coloured glass is brutal and beautiful, rearing up out of nowhere. It's precisely the kind of moment which would be unthinkable – impossible – in the waterlogged sound-world of most modern pop.

Yet, as the album rolls on, it's easy to feel bulldozed by MPP also, since it is, in its own way, somewhat overpowering. The dazzling brightness of the sound is unrelenting – is it possible for ears to squint? The vocals are, by AC standards, clear and prominent, but their frequency range is flooded with a barrage of glimmering samples, crowding the picture. It's a lovely effect, on its own terms – everything shiny with stuff that's dripped off the moon – but this whirling, shivering sound, with all its ripples and disturbances, wears you out. Half an hour in, with too few pauses for breath or reflection, one begins to hunker down. Listening to the three-track sequence of 'Bluish', 'Guys Eyes' and 'Taste', the album's clogged nadir, feels like walking into the wind.

So let's be clear, here: Animal Collective are to be treasured, for their finest minutes, and their attempts to approximate the messiness of being alive and open wide, feeling and reeling and squealing. It's just that Merriweather Post Pavilion, for all its intermittent majesty, is... well, a bit of a mess. While its lyrical themes are uncharacteristically clear - child-rearing and home-building, mostly - a good half of MPP is so vague and sprawling, sonically, that on an instinctive, emotional level, you're unsure how to respond. Within a minute, it might evoke an August sky, a city night, a childhood Christmas – each impression vivid but maddeningly vague, storm-tossed and illogical. At times, it has the frazzled feel of a third straight day on LSD, perceptions whipped to a mousse. Too much!

What's so infuriating is that when they do it well – that is, when they concentrate, and when their music is at its most concentrated – they are now quite staggeringly good. 'Also Frightened' is a neat row of hooks, divided by blasts of multi-tracked vocal (nicked from the last half-minute of The Beatles' 'I Want To Tell You'), and a chorus that looms and settles like a wave. The lulling 'No More Runnin' could be Dennis Wilson's dying plunge. 'Daily Routine', the best thing here by far, is just incredible: Panda Bear sings of a long lie-in, and the track seems to yawn and dissolve, drifting back to silence through long, slow moments of near-intolerable beauty. These tracks are Animal Collective beyond their best, touching true greatness, and a whole disc of this would win the 21st century.

Elsewhere, though, too much scribble and whimsy. 'Lion In A Coma', with its daft pun and its didgeridoo, is twitchily poptastic, almost to the point of quirkiness (saved by a discordant wash, which adds some depth when your lip starts to curl). 'Summertime Clothes' is another shot at a semi-conventional pop song, schaffel and all, which doesn't quite come off - it's catchy alright, but no more than smudged Goldfrapp. 'My Girls', a father's wish dragged through endless reverb and repetition, turns on a small circle indeed, and is ultimately rather boring. The closing track, 'Brother Sport', offers a mash-up of house beats and homeliness which, save for its thunderous breakdown, sounds like a sea shanty for skunked-up landlubbers.

MPP is a logical development from Animal Collective's earlier recordings (the sketchiness turned lurid, the warm guitars frozen out), but as their sound expands, it grows indistinct. It makes me think back fifteen years to the half-forgotten maelstrom of the half-remembered Disco Inferno, whose primitive, seething sample-scapes were equally intense (and far more intimidating), but rarely lost their grip. DI's teeming rain of light evoked the anomie and terror of urban living; Animal Collective, with their earflap hats and droopy eyes, are far less troubled and far more self-absorbed. Perhaps this explains their tendency to wander, to please themselves to the point where it ceases to be constructive? Dropping into this record can feel like walking into someone else's dream, all lit up with a significance that somehow fails to connect.

I'd love to say that Merriweather Post Pavilion was a giant leap - for Animal Collective, and for pop music in general – and yes, much of this record is wonderful. Many will be astonished, and they won't necessarily be wrong; it's a far better and more useful record than most, and its power is undeniable. Others will tell you that this is an important record; I might too, were I still a crusader. But in truth, the best tracks here expose the rest as shapeless and sloppy, and when you sail this close to genuine greatness but are waylaid by your own indulgence, you must expect some censure. A record well worth hearing, to say the least. A masterpiece? Oh, I wish.