I took WREB back in the day and they told us statistically that the pass rate was 90-95%. Out of a class of 100 students, the odds are in your favor! I think the pass rate for my class was around 95% as well.
Not 100% sure but I know California was experimenting with a portfolio based process versus a board exam.
I had a few friends that went through ADEX and it sounded like the pass rate was lower.