I believe the pass rate for assessment attendees is typically about 50% for FOs and 20% for DECs, but it will vary from course to course - they're looking for a standard, not a proportion of each group, so you're not in competition with each other. It's not a hollow statement when they tell you that they want all of you as long as you are of sufficient ability; they have invested time and money in bringing you in for assessment after all.
I think the main cause of failure is CRM related; the play group stuff may seem a little silly with the scenarios they give, but they are very effective at revealing character to the assessors. Some have problems with the computer testing or the sim, but I think that is a much smaller number, and the medical and psyc stuff is thorough but not like NASA/China.
I'd rather have able colleagues rather than lucky ones, so I'll wish you an unconventional "good show".
