Literally an apartheid state and we're rooting for characters that are committed to "keeping it safe" (this means further militarizing the border between Piltover and the undercity and killing children on accident and feeling really bad about it). There's plot elements that expose how bad Piltover is for undercity citizens, but that's just the other half of both-sidesing the issue and calling it a cycle of violence. No, the REAL and IMMEDIATE threat is Silco, the guy actually committed to resistance against apartheid Piltover and even the Good and Right characters are opposed to him - to the point that they're more willing to work with Piltover, the state that has been oppressing them their whole lives and which they constantly talk about how its oppressed them their whole lives, to take him down. There's even the gem at the end where Piltover is on the verge of granting the undercity sovereignty and freedom to end the fighting, but then someone decides to fire a fucking rocket straight into the room this decision is being made in. It almost feels 1 for 1 equivalents being made to Israel and Palestine based on hasbara narratives.
I think the bigger issue is that we so rarely get to see a "clean" revolution where the opressed class is led by people with good motives. That's not on Arcane specifically, but there is a lot of media where the scrappy upstarts sucessfully overthrow power only to have the new power be just as evil, but maybe in a different way (Hunger Games is a notable example).
Arcane plays into that ttope somewhat, but I wouldn't say it plays it straight
They always have to kick the dog, you can't make the revolutionaries too emphatic.