The fact that a character provokes such a strong emotional response proves they're well written. Just because you hate a character doesn't make them poorly written; quite the opposite. It means they're real: a complete and complex person you disagree with so strongly it haunts your every step of the game. That's excellent writing. Now if you hate them because they're as complex as stale white bread and provoke no emotional response at all, that's different. Characters should make you feel. Sera does that. So does Vivienne. You shouldn't expect to "like" every character any more than you like every person you meet in real life.
Seriously? I've been more stimulated emotionally by B level horror movies. Sera's extremely two dimensional, worse than that she barely makes any sense. I literally can't follow a conversation with her, I genuinely don't understand what message she's trying to convey to me. I don't know about vivienne though, don't think I've met her yet. I might not like everyone I meet in real life but usually I can understand them and tolerate them.
I don't particularly like Cassandra, but I can still respect her, and better yet, I can communicate with her on a meaningful level, and that makes me not hate her so much, because even if I disagree with her past and some of her decisions she's worthy of listening to and someone I can learn from.