Quick test for you guys

I tested Edo Hiruzen damage with using Danzo lvl 8 card (+20% ninjutsu buff) and using Gaara lvl 7 card (+17% ninjutsu buff)

The cards from ninja assist system do actually buff only the ninja based on attribute, it doesn't matter which element his skills use.

So the result is that it's an overall buff for that ninja element and not his skill elements

Clear difference can be seen from tests below:

Danzo

Gaara