It's not that buff are "bad", but the fact that they are additive (and not multiplicative) make them less and less useless the more you have. And that's also why elemental attack boosts are good, because they are multiplicative with character attack boosts.

Also, do we know the exact effect of Mordred's Vicissitudes of Fortunes ? If one lands, it could make a high-level Ultimate fight way more easier.