The Canadian prosecutors would have the same concerns that US prosecutors have now. If a prosecutor presses a case built on circumstantial evidence which is essentially bullshit in court, a good defense attorney will hand the prosecutor his ass on a silver platter. These cases get pleaded out in the early stages. The vast majority of all criminal defense work I have done in the past (admittedly a small part of my practice) has involved overcharged bullshit, based on circumstantial evidence, and most of it got nolled (that means the prosecution was dropped and the client was put on conditional or conditionless probation). Usually I only took those cases because I had a lucrative personal injury case with a client who got busted, or his kid got busted, and I was paid for the criminal case out of the proceeds of the personal injury case, on top of what I made as a fee on that personal injury case.
On the other hand, if the prosecution has a solid witness, the case has legs. I attended a murder trial in Connecticut about 20 years ago, as a spectator, and watched as the defense went down in flames because the prosecutor's star witness, defendant's mistress who was given an immunity deal, testified she helped the defendant strangle his wife. She held the pillow and he pushed it. He went down, big time.
Bottom line on prosecutions against johns is you need a live witness, whether it's an undercover or an escort. Escorts are not showing in court and undercovers can be screened for. If you do not have a live witness it would be very difficult to prove anything in court. Cases do not go to trial and are usually not even prosecuted if all that there is is circumstantial bullshit.
I listed to Peter MacKay yesterday talk about this proposed law and I heard absolutely nothing come out of his mouth as to how they planned on enforcing it if enacted. I have been told that this whole presentation was an exercise in electioneering by MacKay. I don't know enough about CDN politics to agree or disagree with that.