If your Holy Radiance seems to be healing for less than you think it should, the good news is that it was an unintended bug that was causing it to only heal 5 players. There is a bug report here if you are wanting to know specifics.
[quote]Fixed an issue where Holy Radiance may sometimes heal less players than intended.[/quote]
There is plenty of speculation over the “sometimes” part, but the good news is it is hotfixed