The difference is that in CC, nobody's optional, so that dev can do things with the story that Carib just can't. Vicky wants to join the harem, in part, because she's jealous of Lucy. But if the player's some kind of deviant who doesn't like Lucy and avoids her route (if that were possible to do), the dev has to account for both paths--which quickly becomes an exponentially-increasing number of paths. In CC, the dev can write one story in one way, but this makes the "player" more of a "reader."
In Eternum, if you're some kind of bizarre freak who doesn't like Luna, you don't have to be on her path (and how much of the purpose for that is "you can avoid LIs you don't like" and how much is "have some kind of game play element," I don't know--though I have my favorites, I can't really see how anyone could dislike any of the LIs in Eternum). But that means either (1) each LI is in her own silo, her actions and motivations independent of all of the others; (2) Carib has to set and keep track of a lot of tracking variables, and write lots of variations of dialogue to account for all of them; or (3) some combination of these. This gives the player meaningful choices, but it's a lot more storytelling work, particularly when (as in Eternum) there are multiple off-ramps to any given LI's path. Carib's done that work, to a large degree. But if you're off Luna's path, Luna's still going to be in the story--and if you're off her path because you don't like her, you probably aren't happy about that.
I don't know that I'd say it's impossible for a game like Eternum to hang together as well as a game like CC, but I'd think it's exponentially more difficult.
And yes, both the "deviant" and the "bizarre freak" are (mostly) tongue-in-cheek.