Is it tho?
I mean ...
On one hand we have Shar temple filled with enemies that reflect radiant damage back to you ...
(
and it can hurt, especialy if you are careless paladin. 
)
On the other we have Death Domain cleric that have ability to bypass Necrotic resistance ...
Yes it is. Because in the specific circumstance of the small part of the game where Radiant damage isn't literally the best damage type in the entire game... Light Domain can just use Fireball and Wall of Fire and burn everything to the ground.
The rest of the game it can run around using Radiant damage with all the gear that makes Radiant damage really good and doing massive damage to all the enemies that are weak to Radiant damage (Yes, Death Domain can also do that too... But then they'd be ignoring most of their domain bonuses)
