USWA1 overpredicts, but just by a bit. No more than 10 pts I'd say. But for some people they are dead on.
USWA2....well this one can over predict by an enormous amount. I've seen one person for whom it overpredicted by 30 points. For others though, it can be dead on.
They definitely dont underpredict though, so if you get a low score, don't blame it on the test.
the questions are in general easier that the standard UW questions, but that is because they try to make it like NBME, which has easier questions as well.
So yeah, if you want 200 more questions to practice with, get these. If you want to predict your score, nbme all the way.