Yes I sometimes use kava and it helps with sleep. That part about the liver was put out there to scare people, I think.
I've drunk traditional Kava - made from the root...tastes like muddy water and it is intoxicating.
I have no reason to doubt that, in whatever form, it may have an effect on the liver if you take too much, use it for a long period, or already have some liver damage.
It may also interact with other meds and it's not recommended in pregnancy.
It's been a real problem among some indigenous communities
Alcohol and Other Drugs - Kava
Common sense suggests the powdered form is not as potent or it wouldn't be for sale anywhere.
But like anything, just because it's natural doesn't mean it's intrinsically good.
D