While not really impressive- Susanoo's subskill is actually a fairly okay multiplier for it's type. Theres no real restrictions, and it's skill boost instead of Damage. It's flexible, and sizable- especially on an SS card that will boast more stats than similar S cards, infact, he's arguably going to do more than most 25% S cards flat out!
The problem is self-enrage is VERY nasty, and more importantly not avoidable, avoiding slotting Susanoo would be preferable, especially with a sizable and farmable list of sword subs.