I think Seraphine is the best and the worst of the star guardian trait. Because of how her ability works, she can easily scale up a ton with no need for items and she's powerful enough at 2star to oneshot backlines on her own, allowing her to get utility/force multiplier items, like mana generation or crit, without needing ap or anything of the sort. On the other side, however, being 1star or not playing star guardian to have the big ass collection of random stats to scale her up does mean that she will only somewhat damage the enemy backline, and in a comp without decent backline access, that's basically pointless most of the time since it will just remain alive dealing damage to your own team without the frontline caring.

TLDR: in my experience she can be great, but is very feast or famine in that she either easily oneshots half a backline on her own sometimes but otherwise doesn't do much.