I think the camaraderie of the characters is so strong that single episode training filler or "Prickly Queen fails to tell Yami she l-l-l-l-likes him" slice of life can be really entertaining. Filler arcs have the potential to be really cool, like some filler dungeon or filler magic zone or something, but trying to tie it into the main plot when it can't move the main plot forward at all seems a bit like a mistake.
(Then again endless anime probably shouldn't exist these days anyways, but yeah)
I'm cooling on the plot but the action is so fucking cool in this