ybrotes_Sargon
Restricted
Nihilus has never been shown {to my knowledge, at least} to successfully drain a prepared adversary. The population of Katarr were oblivious to his arrival, Kreia had to split her attention between both him and Sion; and he immobilized the Exile and her companions before attacking. When Kreia utilizes the attack, it takes four seconds to kill the Council members, which should be sufficient time for an adequately skilled opponent to react.
All this aside, if Tobin is to be trusted, Nihilus's perception is too nebulous to perceive singular Force users. Should Dooku or Sidious be powerful enough in the Force to warrant his attention, they can conceal themselves in the Force to reduce their potency, shielding themselves from his vision and applying their superior skill against him. Likewise, it's highly improbable that Kreia or Nihilus would successfully wield the attack against Vader, who is capable of resisting the Dark Reaper's effects.
Their most powerful abilities negated, Nihilus and Kreia will be easy prey for their many enemies.