I'm not so sure I'd call it underpowered, but yeah it's not like you're going to be able to MC everything on the battlefield in 1 turn like you could in vanilla Xcom. I think the highest VooDoo skill I've seen was on a slave soldier who was at 70, but screening slave soldiers is kind of detrimental as you lose the slave. I've seen peasants with high 60's and they're cheap so they're a lot more screenable without hurting your resources. All the pirate gals, hands, warriors, vetrans, escaped lunatics cap out in the low to mid 60's, Freaks can go a little higher than the rest but screening gals is really expensive (especially if you keep too many around and your officers get promoted really high). Lokk'naars cap out around 50. Then you have to take into account the skill caps. VooDoo equip uses a formula using power and skill so you really want both to be high. All that being said you're highest raw power/skill will come from Peasants and Slave Soldiers but they can't wear any of the fancy VooDoo armor, they can only use the various devices. Something to keep in mind as wall is that armors can raise and lower VooDoo power.
As far as mitigating stun, energy, bravery, etc, effects from using VooDoo skills, there are a bunch of different consumables that you can carry around and use as necessary, Finely distilled rum is a nice portable "all in one" option with a decent amount of uses per effect.