The damage from radiant destruction starts to increase at 50% and reaches its maximum at 0%. So basically to get the full 300% bonus your enemy needs to be pretty much dead.
There were issues with other multiplicative stuff like vulnerability, but that should be fixed now. Biggest problem of the skill now is, that you cant weave while channeling. Atm light attacks do a lot if damage, which you dont want to miss. Radiant destruction is a dps loss atm, since the light attacks weaved in a rotation are too essential atm.