Not every mystery has an elemental attribute, even if is full nin. There are pros and cons. The pros of having the attribute is that it gets the boost if the enemy you hit has a weakness towards that element and that gets boosted by the new feature related elemental exagon. The cons are they are subject to the enemy elemental resistances. Generally speaking is BETTER if they have no elemental attribute because generally the elemental resistances decreasing effect is bigger than any boost you can give it linked only to elemental effects.

As soon as we get unlocked normal and hard mode of kage TI (i sincerely hope soon) you'll better understand what i mean because +4 elemental resistances on tools will be again available (and you will also see how edo hiruzen damage falls down against somebody with +40 resistances while AGK damage stays the same since has no attribute).