All you have to do is look at the expert evidence in both Brown and Chan.
In Brown, the trial judge decided that, although Mr. Brown testified that he had consumed between 13 and 17 drinks and snacked on handfuls of magic mushrooms all evening, it was unforeseeable that he would lose the power of self-control. We're not even talking about harming someone.
The cases before us demonstrate that this is going to be really difficult, because the defence can get experts who are prepared to say that the person was in a state of extreme intoxication, but they're not prepared to say—or they feel they cannot say—that it was foreseeable because they don't know exactly how much he consumed. They don't know how much psilocybin was in that particular dose of magic mushrooms. They don't know the rate at which he consumed. They don't know his experience with drugs.
All of these factors mean it's going to be very hard for the Crown to prove even foreseeability of loss of control.