ATLA was a children's show that could be enjoyed by adults, given that back then Western animation was seen as for children's first and foremost so they had to keep threading the line.

Arcane is more like 16+ because the creative creators are also a pretty huge company with the resources to put out whatever they wanted from the get go.

Also Avatar as a whole is just a much more optimistic story with mostly good people at their core as characters, Arcane is just bleak tragedy all around filled with damaged people in a broken world.

Probably give a try to Castlevania if you haven't, there's maybe a little too much silly banter between characters at times but beyond that its pretty bleak and tone cohesive.