Silas x Felicia rather annoyed me.
I'm alright with Silas x Azura and Silas x Mozu but I just don't like Silas x Felicia at all. First he brought up that rumor to her which he should have just kept to himself or asked the Avatar before confronting her about it. While he does apologize for it, he apologizes by "teaching her to be less awful". Also, (and this goes for most of his supports), he's completely obsessed with the Avatar, but then with the Avatar themselves, he acts completely normal. I love him and all, but man does he have some weird dialogue.
I didn't like the Keaton x Avatar support (at least in Japanese, haven't actually read the English one yet) with the whole "treasure" thing but that might just be me. Kinda a shame though, because I liked him as a character a lot but reading that support made me dislike him a bit. His support with Nyx kinda redeems that, though imo.
Beyond that, I don't know. Most bad ones I either haven't read, or have been elaborated enough on here.