thats not true at all the reason castoria is so busted is because she broke the looping meta by enabling every arts character in the game to loop plugsuitless with black grail. and protomerlin is just a sidegrade.
as for skadi/ruler skadi, quick is still outclassed. buster comes close but still isnt better than arts because oberon teams require plugsuit. while vitch is pretty good she still cant enable every buster servant like castoria can enable every arts servant.
castoria is literally the best character in the game, the only mustroll in fgo