I honestly think Seraphine is one of the most fun to play champions in League just because she has so much flexibility in her kit which allows her to adapt to any situation and she often feels like an entirely different champion from game to game.

Great mixture of both utility and damage.
Terrifying CC combo that keeps enemies stun locked.
Squishy herself, but her W does wonders for survival.
Great synergy with any kind of team comp.
New item update synergizes extremely well with her kit.

As for a few things to keep in mind...

Still squishy and easy to focus down in teamfights.
Not as accessible as you may first think.
Isn't the best enchanter-style support due to very long W CD.

No matter where you go, there is no general consensus on which Seraphine build is the best. U.GG advocates for full support, while some guides promote all damage. Here I will break down just why hybrid Seraphine is the most consistent from my experience.

Still deals VERY strong damage [on par with most of your team].
Still receives loads of ability haste to keep her cool downs manageable.
Does reliable poke in lane but also scales extremely well.
Bone plating + revitalize is unparalleled in terms of survive.
Other secondary trees don't capitalize on Seraphine's kit as much.

This of course isn't to say don't try out / take other build paths; Seraphine is all about flexibility, use that to your advantage. You should always be switching out your runes, summoners and item builds from game to game depending on what you feel is needed best [or simply what sounds the most fun at the time~].

This is a very hard to answer question, as Seraphine is very much like three entirely separate champions depending on what build you go. But one principle that stays the same is capitlaize on the strengths of the build you are using.

Hybrid Seraphine:
Fulfill every role. Poke every time your manaflow is off cooldown. Early game you are a massive threat, so don't be afraid to bully out enemy laners. Revert to playing like a pure support in the late game however, staying in the back and landing as much CC as possible.

AP Seraphine:
P loads in lane. Look for kill opportunities, land sneaky Es, ping reliably to communicate to your ADC when to apply the pressure. You shine in the early game, so use that to get a lead. After that, revert to playing in the backline during teamfights.

Support Seraphine:
Play very safe, however, seem over zealous with your poke. Although your spells won't deal nearly as much damage as with other builds, you still get gold from spelltheif's edge. You shine in the late game, when you and your ADC scale, and you are able to provide them ample opportunity to do what they do best by using your CC chain and ALWAYS using your passive echo on W for a shield.