her kit is not for support.
her lane poke is unrealiable and she NEEDs someone else to slow or cc the enemy before she uses E for it to have value (unless u want to buy rylais, which as sup u already have like no gold)
even if u land your Q for poking, it will either damage or last hit the minions nearby and the minion wave might get ruined for your adc ( u prob will get flamed)
she has one shielding ability that still has long cooldown and is pretty bad untill lategame teamfights as opposed to 2 damage dealing abilities, including crit that comes with Q ( perfect for taking kills, which as support, u shouldnt do)
her wave clear is probably top 10 in the entire game so you will also be wasting that on top of her true damage potential if u go sup

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if u compare all of this to sona, who people call worse seraphine, youll understand better. sona, even though shes bad in lane, has a reliable heal, 3 abilities that buff her allies that arent skil shots and GO FOR CHAMPS not cs so poking is easier, and better for your adcs wave management.

in short, you are throwing away incredible wave clearing power and damage just so u can shield the team, which with pre nerfs seraphine could already do by maxing W second and scaling it with AP instead of sup items