he is actually pretty good. the first healer that can trigger attacks. the changes to the healers siena and lucille especially, with their heal on attack passive changed is mostly due to kahuna i
believe.
take a look at the formation above, if you change shizu with kahuna spiritwalker, freya with another gunslinger (with good crit rate) and teressa with the old passive sienna/lucille that can heal on attack, you can form a party that can trigger a lot of auto attacks in one turn. in a offense oriented team, he is still pretty strong. you sacrifice some heals for more trigger attacks. if you play a witch doctor kahuna, you can just put in a life drain gem and put him in between a gunslinger or warlord, it will enable him to strike a few times a turn when you are lucky.