Yeah, I think all of them are some combination of untrustworthy (Martellus and family) and/or trouble magnet (Agatha), and they're all so powerful that those traits make them very dangerous to have in the city. Also Agatha has The Other's voice, which brings up the possibility of her commanding the Revenants within Paris. Even if Colette trusts her, she might do it by accident or Lucrezia-in-Agatha might get temporary control, and that's a serious risk.