Just in case you don't follow the BugTracker regularly, I want to let you know that I found a very significant problem with YafaRay lighting affecting Path Tracing and Photon Mapping. See: http://www.yafaray.org/node/803#comment-1850
Hopefully this will be solved in the next version, as explained in the bug tracker. However I wanted to inform you of this, because this has been probably happening for years and went unnoticed until now, causing subtle (or not so subtle depending on the scene) artifacts that reduced realism in the scenes.
This fix is still experimental, but so far it looks like it's working. We will have more noise as the light samples will be (correctly) more uniformly distributed through the objects, so they will be more separated, but the lighting should be ok with the fix and eventually with more samples it should converge to a correct solution (as opposed to all affected YafaRay versions which would never converge to a correct solution due to this problem).
I hope it helps. For now we will test an alpha version with the fix in a small circle of developers and alpha testers and if it's ok, we will distribute a beta version in the next few weeks.