The effect simply removes the ability of choosing which card you wish to revive when playing a medic instead making the revived card randomly selected from the discarded pile.
While the effect can be considered a double-edged sword it is not entirely true. Interestingly hero medic cards like Yennefer and Menno Coehoorn are immune to the effect and will allow the player to choose which card they wish to revive when played.
The card is very effective when playing against the Scoia'tael deck as it will prevent the opponent from reviving the spies you may have used in a previous round. Since the Scoia'tael deck relies heavily on muster cards and medics it makes it almost impossible for them to revive the right card.
Unfortunately the card will more often than not have no effect on the opponent's medics and only effect the players medics. Making the card a self-damaging effect.