The story was a major US intelligence bureaucrat organizing a major terrorist act on US territory to profit wealthy friends and manipulate policy. That story is hardly "relentlessly pro-imperialist." The terrorist is also successful, so the hero=winner criterion is violated as well. Rubicon can be justly criticized as a shallow view that blames imaginary rogues, creatures likely as mythical as witches, instead of the system.
These characters are naturally unquestioning of imperialism as a system, and they are pretty much indifferent to the consequences of their actions. When they aren't, they fall back on cliches. Although the viewer may choose to look only at what the attractive characters say, the series shows a quite different view. Tonya's drug addiction and Miles' grotesque lapse of security go unpunished, revealing an ingroup mentality that contradicts the self-image of professionalsm, even if they don't question their good luck.
When Will freaks out at being surveilled and bugged, we can (if we choose) understand that Will's hapless victims felt the same.
And when the boring lead is horribly shaken at killing a man, self defense or not, the show is saying something about the value of life and the gravity in killing.
Sepinwall's extended defense of AMC's rightness was distasteful. Looking back at his Rubicon reviews, his criticisms of the finale (and retroactive distaste for the series as a whole,) seem rather niggling. Since he didn't even comment on the success of the terror attack, despite its rarity in thrillers, perhaps he couldn't articulate or even analyze the source of his displeasure?
One of the points he mentions, Will's success in killing Donald Bloom instead of the other way round, hinged on a major plot point very elaborately staged indeed. Namely, Will had a gun that David had smuggled to him in a motorcycle! After such an ostentatious gun on the mantle intro, surely he should have noticed?