|• Spell damage reduction 95%|
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They have very low defensive stats for a level 6 unit, the lowest health value if we exclude the Enchanter and also the lowest defense.
The Diamond Golems also have the lowest attack and damage values of a level 6 unit.
Efficiency and cost
Because they can be recruited pretty easily after taking a Golem Factory, and also due to their very low cost, they are units worth having in early game. However, later on they serve only as cannon fodder.
Since they take only 5% damage from spells, they can be used by a hero who has learned Armageddon, allowing you to sometimes wipe out your enemy's army in one spell, without hurting your own stack too much. There are much better options available out there (Namely: Efreet, Black and Gold Dragons or Phoenixes), but Diamond Golems can be an option if need be.
If you have a stable access to their dwelling and enough resources, you might consider placing them into a town's garrison (but beware of morale).