The plaintiffs would have to prove that the videos that can be downloaded with youtube-dl are protected by an objectively effective technical measure and that the tool circumvents this protection, that the accused was aware of the effectiveness of the measure and the illegality of his actions.
I studied law at the University of Zurich (Switzerland) and would be calm in face of such a criminal prosecution. The plaintiff's risk of litigation would be considerable and the defendant would have little to fear.