I'd say a lot assert their artistic quality (btw I used the "hold" phrasing off you, so these quotation marks are odd); movies about the creative process frequently leave room for a reflexive reading where the movie itself stands in for the work the subject does. I mean, we're talking about Birdman in here. Examples are all over.
When I said "hold", I was talking about the film's champions, not the film itself. The people I've seen praising Boyhood the most profusely have been people who come from a similar milieu it portrays and have some level of nostalgia for it, in the same way that American Beauty was most lauded by jaded suburbanites who resented its flaws and limitations.
Anyway, Boyhood is factually not straightforward in terms of editing and narrative structure. it's replete with ellipses and hanging plot points that do not belong in a traditional coming of age story. And I said it doesn't find its setting inherently profound. By that I meant that, while American Beauty tells you the banal is deep but doesn't make a convincing case and is shallow the way you describe, Boyhood is shaped cannily so that capturing an everyday life illustrates how children form personal ideologies in line with/in contrast to the actions and thoughts of those around them.
It's about as straightforward as I could imagine a movie about 12 years in a boy's life being - age 5, then age 6, then age 7, and so on. I applaud the movie for eliding some of the traditional points focused on in such films, but the problem comes in when you don't introduce much else of note to replace the things you elide. Ozu was an artist who intelligently elided the most melodramatic and shallowly emotional parts of the stories he told and substituted them with poetic imagery and with scenes showing smaller moments that core more deeply into the complexities and contradictions of his characters. "Boyhood" elides some of the typical cliche moments, but it replaces them with moments that are, for the most part, all surface. There's a 7 or so minute scene where Mason talks with a girl from his class as they walk down an alley, and they don't really say anything particularly memorable to develop either of their characters, and all you really get from the scene is exposition about where Mason is at that point in his life and a fairly banal, overlong depiction of the dull little moments that adolescence is so often composed of. There's a scene where the kids are sitting around bullshitting in an unfinished house, and all you really get from it is that kids are bullshitters. There's a scene where you just watch kids getting their copies of Harry Potter from a bookstore, a scene where he stares at a dead bird, etc.
There are moments of the film that are effective. It's a good choice for the film not to show the mom's second boyfriend in an alcoholic rage. You "get" that she's fallen into the same basic pattern, and there's no need to dwell on it. It's a good choice for the movie to not show Mason breaking up with his girlfriend, since that doesn't really matter as much as the inevitability of it happening. But for everything the film does that's effective, there's probably 5-10 scenes that go nowhere, have little depth (either in the moment or in the greater aggregate of the whole film), and are fairly rote in the way they are shot and written.
Sure, there is what you described - you see what surrounds the kid as he grows, and you see what he ends up keeping, what falls away. You have the well-done arcs of Arquette and Hawke. But in the end, the overall posit of the movie - that life has no narrative nor point, that it is primarily composed of small, seemingly inconsequential moments that stick rather than the typical big moments - is something that can be gleaned in the first 45 minutes or so. After that, it's just scene after scene. It's not "slow", exactly, for the episodes are mostly short and diverse enough that the whole thing never quite settles into a rut, but it never cores into anything to any significant extent. He gets called a fag at one point, his mom is reading to him, he's visiting a college with his girlfriend. It's "realistic", in a shallow kind of way, but it's mired in the main actor's increasingly stiff and wooden performance and the generic shallowness that Mason is composed of at an almost atomic level.
The movie has plenty of good parts, but one has to ask if the story, such as it is, would really work or attract any interest in the absence of the central hook that got it made in the first place. It's very neat to see people age before your eyes in such a compressed span of time, but outside of that initial pang of emotional resonance (double for me, since I grew up in the milieu the film depicts), does it have much else to offer? A little, yes, but not nearly as much as its reputation might suggest.
Edit: "Saving Private Ryan" is more "what the fuck was anyone thinking calling this shit good?". "The Thin Red line" is the movie from that year whose lack of recognition by the Academy will befuddle film enthusiasts.