Everyone I know who's been to an assessment say that it places a very heavy emphasis on non-technical skills, personality and attitude. They don't ask for any alien level of technical knowledge (which some other companies do), but the HR part of things (individual and group) can make the deal for an averagely knowledgeable and experienced candidate or break it for a highly experienced one who knows the FCOM by heart. I know pilots with over 10,000 hours who failed and pilots with 300 hours who passed.