My complaints actually have nothing to do with the film's realism or lack thereof. I like it for what it is, but I haven't really scratched the surface of my problems with The Untouchables' story or character, which all felt to me very thinly drawn, realism or no. De Palma and Morricone make this movie, though. Without them, I could easily see this being shit. And really, the film is worth watching for De Palma's iconic shots of Chicago and Morricone's score alone. The exhilarating set pieces are a bonus.
My main problem with this film is that I never really felt like I was rooting for Ness's squad, just against Capone. The only good guy that had any depth was Connery's Malone character, with everyone else just basically being simple types (to be fair, Malone barely escapes this). Ness himself is also a pretty weakly drawn character. He says how he's "broken every law" to take down Capone, but this never really registers. He participates in the violence pretty gleefully, and he only really has one moment of doubt, and even that feels dishonest (and is invalidated a moment later when he throws Nitti off the roof). The film never really earns any sympathy for its characters, just kind of throws them at us and makes us accept them. They're the good guys, and we want them to win. Simple.
If we're going to compare this film to "mythological" westerns, I guess that I'd have to say that the best films in that genre earn a certain amount of investment in the characters on the part of the audience. Heck, even most, if not all, myths have some pretty complex and engaging characters at their centers. Hercules and Oedipus are more than just "types," for example, and I think letting stories of any kind off the hook for employing "mythological archetypes" is kind of lazy. You have to add to the archetype, which The Untouchables never does.
To me, this is what's holding the film back. It's an 8, easy. I use the word overrated because my friends kept telling me The Untouchables was a 10. It was a fun ride, with some great examples of "pure" cinema, but it never drew me in as anything more than a mere spectator. And that's one of my personal criteria for greatness, hence my stopping at "Really Good."
And, to be fair, pretty much everyone in this thread has the right attitude towards this film, so far. So, I guess I should say that The Untouchables is overrated in my personal frame of reference, with regard to what many in my real life circle of friends have said about it.