As reported by surena in http://www.yafaray.org/community/forum/viewtopic.php?p=31423#p31423 when using Bidirectional with World Background there are problems.
Based on what surena reported, I did several tests and found these issues:
* When using single color/texture background, the background color/image does not show in the specular/glossy reflections at all.
* When using sky background, YafaRay crashes.
I've tried old YafaRay versions and same happens. So, this problem seems to have been happening for a long time. I will investigate.