I think it is important to understand that Riot's brainworms tend towards the liberal "NUANCE" variety, rather than the idealization of any given faction. Runeterra isn't really supposed to have "good" factions who are purely thus from my reading. Rather, the appearance of good is generally a strong indicator of the presence of evil, whether well-intended or not (we find the standard-issue liberal "road to hell" dialog here, at least implicitly).

Demacia is a deeply authoritarian state that persecutes magic users and maintains an autocracy on par with the supposedly "barbaric" brutality of fascist Noxus (and for all Noxus' flaws, unlike Demacia, there is at least some class mobility in Noxus). P/Z are supposed to be a bit of a dialectic (not that you'd ever find that term in their faction details) about progress, control, and liberalism. It isn't an accident that Zaun takes on both some of the worst elements of libertarianism and an unavoidable form of "class" consciousness (sublimated into nationalism, of course, because liberal ideology cannot tolerate a class formation that is purely thus) and resistance to the progressive technocracy of Piltover. The understanding of that dialectic is, because it is infected with liberalism, fundamentally flawed. Idealism is elevated above an understanding of the material interests of the contending classes, but it doesn't ignore the reality that progressive technocracy leaves a fuckload of people behind in building its "utopia". It just both-sides it to the point of inscrutability.

I haven't seen Arcane (too poor to pay for Netflix in order to watch the three or four shows I'd ever want to watch on it), but what I've read in terms of plot-summaries indicates to me that the audience is exposed to Piltover through some of its most sympathetic faces (Jayce, Caitlin - Vi, to an extent) and their personal stories mean that they elide the darker elements of Piltover. It is worth remembering that Piltover also employs monsters like Camille to "maintain the balance".