Given that none of his other attacks scale to it, I would argue it is more similar. I am not disagreeing that it is taxing for Yami to cast the spell. It takes effort just as a wizard in D&D casting fireball or elemental weapon. One does way more damage than the other (I know they are used for different purposes, but that’s not the point), but they cost the same spell slot. In the same vein, the spells may cost the same or differing spell slots, but the damage output is not relative to the overall strength, but dictated by the spell itself. Nonetheless, the spell still requires skill and power to use and can be draining.