From what I understand, "qualified immunity" has become a legal quagmire in US law. The doctrine necessitates that there be a pre existing judicial decision that THE EXACT SITUATION IN EVERY DETAIL was discussed and ruled on by the courts before a cop can be successfully convicted. If this is true - and I'm just getting info from Twitter-lawyers - then all the cop's defence has to do is come up with 1 small differential detail and the cops walks - although other cops afterwards might be put on notice that this fact situation too has been added to the list by the judges.
And this is open to abuse by the defence because virtually every situation is slightly different.
Canada does not have "qualified immunity". It has the "doctrine of excessive force" - that the cop has to subjectively believe that he is using only a reasonable amount of force to subdue the suspect and that amount of force has to be objectively reasonable as well. It's far less cumbersome and intelligible and it doesn't require a previous ruling on every little detail to put a cop on notice that he could get successfully charged. It's still very, very difficult to get a conviction against a cop in Canada, because people know that cops have to be able to do their job in tense, confusing and stressful circumstances. But at least it avoids obvious abuse by the defence.