The low pass rate I think is more from lack of preparation on the applicants part. Some people still seem to think of it as an the ATP written exam and it is not that.

The test has been in place for three plus years now, so I'm sure there is ample data on it.

I wouldn't be surprised if the fail rate has/will go up now that they are taking restricted ATP's, just because the test is different than what you are exposed to in the part 91 training world and is definitely slanted towards pilots with turbine aircraft experience.