You're undermining the usefulness of a complete AoE resistance buffs. Granted it's not as attractive as Ninjutsu/Taijutsu, etc. But it's still a massive buff to a good stat line.
Not to mention, it's almost certain when he dies, he'll take someone with him. (Unless they're immune to Ninjutsu).

He's a powerful ninja in the right team, There's not much to change about him outside of Enma.