After testing Castoria thoroughly i can finally give a verdict about her and how she change the meta
First of all, let's be clear about one thing, she is not going to sideline Tamamo, the only problem is that even when Tamamo is aimed to spam healings, skill time cooldowns and np drain she is aimed at survivability,the thing is you need to know her thoroughly to use her effectively, that problem doesn't happen with Castoria, you know how to use Merlin or Skadi? you already know how to use Castoria i mean come on just check their icons together
Castoria toolkit same as her teacher is aimed to buff and NP loops, the difference with Merlin is that she cant make NP rain automatically, you need to use Art combos for that pair her with high np gain nukers like Vlad or Dioscuri and with good chains you can spam Kaziki Bley or Dioscuri Tynderae for example in one of my test i nailed this with Castoria at 1 1 1, with her you can remove the limitation of Dioscuri that "they need to be NP2 to not be garbage"
Want to nuke Sabers and shoot NP like a machine gun? pair Castoria with Chloe, want to make lancer cries tear if impotent rage? pair Lancelot with her and watch those sweet numbers go up and up, more if you shoot Castoria np for maximum loops there is a entire thread in Reddit (disgusting i know but i did for you guys so you dont have to) with all the combos that generate NP loops with Castoria, also she can remove the Anti Merlin strategy
Now come the parts where i cool the jets of the Tamamo fags, what are the things she CANNOT do, first of all she cant heal, like at all, nope, zero nada, mind you she can't heal ALONE but guess who is one of the better partners for loops? yeah Snek boi, of course is not as efficient as having one servant to do it like with Merlin but the option is on the table, another thing she can't do is that her toolkit ditch anything related to criticals in exchange of NP gain, of course this is because if you are going to use her at max efficiency that means you are ditching Quick for Arts so is not needed, Buster benefit for this because there are servants like Mordred that have things like Cigarette lion
She also dont have the auto recharge of Merlin, but again if you are using her that means you are going full arts, her gain is enough for the loops, btw something i noticed that was her NP is oddly the bane of Rhongomyniad, in game the spear of lancer artoria come with invul pierce, she comes with invul pierce protection, and she appears in the next lostbelt, we saw someone using Rhongo on atlantis, there is a pattern here