Long list, sorry- and I'm sure I forgot some
Archanea: Merric x Elice (no real reason, I don't much like Elice in general tbh), Camus x Nyna (I don't like the premise, and it's doomed anyway. I have plenty of better Nyna ships)
Valentia: Alm x Celica (it's boring. They have better chemistry with others), Gray x Clair (no)
Jugdral: Lewyn x Erynis (don't like their conversations and overall dynamic), Finn x Lachesis (Finn has better options and I don't like what ship fuel there is for them. I also prefer Finn as Nanna's adoptive father), Leif x Nanna (they're raised closer to siblings than childhood friends imo)
Elibe: Roy x Cecilia (no), Eliwood x Ninian (don't like their dynamic, and the ending is tragic for Ninian), Hector x Florina (no), Priscilla x most people (I don't like her), Lyn x Hector/Eliwood/Rath (I don't see her being happy at court, and Rath is super bland)
Magvel: Colm x Neimi (don't like their dynamic), Artur x Lute (same as above), Innes x Eirika (it's funny, but it's not romantic), Ephraim x Tana (I don't read Ephraim as being into her at all)
Tellius: Ike x Soren (it's fine, but it feels like Soren is too dependent on Ike. I also prefer both of them with other people), Astrid x Makalov (need I explain this?), Micaiah x Sothe (please. She raised him like a little brother, iirc he even calls her his sister in PoR)
Ylisse, Fateslandia: combining these because we're entering giant support list territory. Chrom x F!Robin (M! support is much better written), Virion x Sully (no), Nowi x most people (only Donnel and Ricken feel okay), Nah x most people (she avoids the age issue but most of her supports are horrid), Robin x any second gen, Corrin x any royal or second gen (no), Soleil x Ophelia (it's horrid)
Fodlan: Byleth x students (this would be fixable), Byleth x Rhea/Sothis (no), Edelgard x Hubert (I prefer them as platonic found family), Sylvain x any woman (yes I read all his supports, no he's not misunderstood), faculty x students (all of them)
Elyos: Alear x anyone really (lifespan issues leaves four options, three of which are half siblings -the twins being parallel Sombron's kids doesn't make it different in my eyes- and one of which I didn't like the support of)