As tibia.com says:
This bonus only applies to items which can be equipped unless they are limited to one drop per boss kill, e.g. Ferumbras' hat. Such unique drops are not affected by this bonus.
With this knowledge, using boss bonus on bosses like: Zushuka, The Welter or even Mad Mage (who drops mage's cap) is pointless? Or these items are not limited to one drop per boss?