It is not diminishing returns it is cc immunity. They have cc immunity from our skills, but we don't have cc immunity from their skills.
With the CP changes they will need to reclassify what's what as disabling effects e.g. movement reductions, immobilization and hard CC effects.
An example, Robes of Hist getting a jump in value when the heal is decent backed by the CC damage reduction from Update 29 changes.