I've found several issues while interacting with my companion.
Sets that DO NOT work on companions - Bugs:
Minor Toughness (passive ability) - while healed by a Warden, they do not get this buff.
Leeching Vines (active ability) - this is unable to be cast on a companion.
Spaulder of Ruin (set) - they do not get damage bonus increase while within the aura.
Sets that DO work on companions - working as intended:
Ebon Armory.
Pearlescent Ward.
Spell Power Cure.
Powerful Assault.