[@PKMNB0Y]

It's not going to be something that's used often. Voice of Influence won't work on a Servant or Familiar (Like Giles squids or Madea's Dragon Tooth Warriors) unless said servent gets a massive debuff to Magic Resistance and they are idiots with no mental fortitude. It's more for a one time use to influence a magus to perceive what he wants them to, or to easily convince mundane people that nothing Supernatural is going on while Hyde rips someones arm off and beats them to death with it.

Any magus worth their salt, or have the same level of knowledge as Shirou can purge foreign influences by cycling their prana, so as long as another player can figure out something is up or they have a Servant that knows them enough to realize the Master is acting out of character.

To elaborate, even if it works, Voice of Influence doesn't let Richard control anyone. He can't go walk into a theater tell the audience to kill themselves, or place someone under a binding Geaes contract if they didn't believe it was in their best interest in some measure. It's making them susceptible to believe his words if they were uncertain before.